View source for Indigo

{{short description|Color}}
{{about|the color|the dye|Indigo dye|other uses}}
{{redirect|Violet-blue||Violet blue (disambiguation)}}
{{distinguish|Indego|IndiGo|IndyGo}}
{{pp-pc}}
{{Use dmy dates|date=October 2019}} 
{{infobox color
|title=Indigo
|image=File:Indian indigo dye lump.jpg
|caption=A piece of indigo plant dye from [[India]],<br />about {{convert|2.5|in|cm|0|abbr=on|order=flip}} square
|wavelength=420–450<ref name="EoF">{{Cite book|url=https://books.google.com/books?id=HQWNJyRV6kMC&q=%22Indigo%22+++%22450-420%22&pg=PA189|title=Encyclopedia of Physics|first=Joe|last=Rosen|date=26 June 2017|publisher=Infobase Publishing|via=Google Books|isbn=978-1-4381-1013-4}}</ref>
|frequency=≈714–670
|hex=4000FF
|source=[[CAM16|CIECAM16 248:2022]]<ref name="css3-color">[https://cie.co.at/publications/cie-2016-colour-appearance-model-colour-management-systems-ciecam16 CIECAM16 248:2022] {{Webarchive|url=https://web.archive.org/web/20240420151156/http://cie.co.at/publications/cie-2016-colour-appearance-model-colour-management-systems-ciecam16 |date=20 April 2024 }}. J = 41, C = 74, h = 289</ref>
|isccname=Vivid purplish blue}}

'''Indigo''' is a term used for a number of [[hue]]s in the region of [[blue]]. The word comes from the [[indigo dye|ancient dye of the same name]]. The term "indigo" can refer to the color of the dye, various colors of fabric dyed with indigo dye, a spectral color, one of the seven [[colors of the rainbow]] as described by [[Isaac Newton]], or a region on the [[color wheel]], and can include various shades of blue, [[ultramarine]], and green-blue.  Since the web era, the term has also been used for various purple and violet hues identified as "indigo", based on use of the term "indigo" in [[HTML]] web page specifications.

The word "indigo" comes from the Latin word {{lang|la|indicum}}, meaning "Indian", as the naturally based dye was originally exported to Europe from [[India]].

The [[Early Modern English]] word indigo referred to the dye, not to the color (hue) itself, and indigo is not traditionally part of the basic [[color term|color-naming system]].<ref>{{cite book|last=Ottenheimer|first=Harriet Joseph|title=The anthropology of language: an introduction to linguistic anthropology|year=2009|publisher=Wadsworth|location=Belmont, CA|isbn=978-0-495-50884-7|page=29|edition=2nd}}</ref>

The first known recorded use of indigo as a color name in [[English language|English]] was in 1289.<ref>Maerz and Paul ''A Dictionary of Color'' New York:1930 McGraw-Hill Page 197; Color Sample of Indigo: Page 117 Plate 47 Color Sample E10</ref> Due to the extensive knowledge of indigo cultivation by enslaved West Africans, indigo became a major cash crop in the American colonies.

Newton regarded indigo as a color in the [[visible spectrum]], as well as one of the seven colors of the [[rainbow]]: the color between [[blue]] and [[Violet (color)|violet]]; however, sources differ as to its actual position in the [[electromagnetic spectrum]].  Later scientists have concluded that what Newton called "blue" was what is now called [[cyan]] or blue-green; and what Newton called "indigo" was what is now called blue.

In the 1980s, programmers produced a somewhat arbitrary list of color names for the X Window computer operating system, resulting in the HTML and CSS specifications issued in the 1990s using the term "indigo" for a dark purple hue.  This has resulted in violet and purple hues also being associated with the term "indigo" since that time.

Because of the [[Abney effect]], pinpointing indigo to a specific hue value in the [[HSL and HSV|HSV color wheel]] is elusive, as a higher HSV saturation value shifts the hue towards blue. However, on the new [[CAM16|CIECAM16 standard]], the hues values around 290° may be thought of as indigo, depending on the observer.

== History ==

===Indigo as a dye===
{{Main|Indigo dye#History}}
[[File:Indigo plant extract sample.jpg|thumb|left|Extract of natural indigo applied to paper]]

''Indigo dye'' is a blue color, obtained from several different types of plants. The indigo plant ([[Indigofera tinctoria]]) often called "true indigo" probably produces the best results, although several others are close in color: [[Japanese indigo]] (Polygonum tinctoria), Natal indigo (''[[Indigofera arrecta]]''), Guatemalan indigo (''[[Indigofera suffruticosa]]''), Chinese indigo (''[[Persicaria tinctoria]]''), and woad (''[[Isatis tinctoria]]'').

''Indigofera tinctoria'' and related species were cultivated in [[West Africa]],<ref name=":0">{{Cite web |title=Uncovering a Women's History of African Indigo |url=https://www.brooklynmuseum.org/stories/uncovering_a_womens_history_of_african_indigo |access-date=2025-08-01 |website=Brooklyn Museum |language=en-US}}</ref> [[East Asia]], [[Egypt]], [[India]], [[Bangladesh]] and [[Peru]] in antiquity. The earliest direct evidence for the use of indigo dates to around 4000 BC and comes from [[Huaca Prieta]], in contemporary Peru.<ref>{{cite journal |last1=Splitstoser |first1=Jeffrey C. |last2=Dillehay |first2=Tom D. |last3=Wouters |first3=Jan |last4=Claro |first4=Ana |date=September 2016 |title=Early pre-Hispanic use of indigo blue in Peru |journal=Science Advances |volume=2 |issue=9 |article-number=e1501623 |doi=10.1126/sciadv.1501623 |pmid=27652337 |pmc=5023320 |bibcode=2016SciA....2E1623S }}</ref> [[Pliny the Elder]] mentions India as the source of the dye after which it was named.<ref>{{cite web |title=Night of the Indigo |url=https://www.harappa.com/blog/night-indigo |website=harappa.com |access-date=20 May 2016 |archive-date=14 January 2019 |archive-url=https://web.archive.org/web/20190114153312/https://www.harappa.com/blog/night-indigo |url-status=live }}</ref> It was imported from there in small quantities via the [[Silk Road]].<ref>{{cite journal | first1=Robin J. H. | last1=Clark | first2=Christopher J. | last2=Cooksey | first3=Marcus A. M. | last3=Daniels | first4=Robert | last4=Withnall | title=Indigo, woad, and Tyrian Purple: important vat dyes from antiquity to the present | journal=Endeavour | volume=17 | issue=4 | date=1993 | pages=191–199| doi=10.1016/0160-9327(93)90062-8 }}</ref>

The [[Ancient Greek]] term for the dye was {{lang|grc|Ἰνδικὸν φάρμακον}} (''indikon pharmakon'', "Indian [[:wikt:φάρμακον|dye]]"), which, adopted to [[Latin]] as {{lang|la|indicum}} (a [[second declension]] noun)  or ''indico'' (oblique case) and via [[Portuguese language|Portuguese]], gave rise to the modern word [[:wikt:indigo|indigo]].<ref>[https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3Dfa%2Frmakon Ἰνδικός] {{Webarchive|url=https://web.archive.org/web/20210129153557/http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3Dfa%2Frmakon |date=29 January 2021 }} in Henry George Liddell. Robert Scott. A Greek-English Lexicon. revised and augmented throughout by. Sir Henry Stuart Jones. with the assistance of. Roderick McKenzie. Oxford. Clarendon Press. 1940; English ''indigo'' since the 17th century, changed from 16th-century ''indico''.</ref>

In Ancient West Africa, 

[[File:Isatis tinctoria 003.JPG|thumb|upright=0.8|Indigo extracted from woad]]
In early Europe, the main source was from the woad plant ''Isatis tinctoria'', also known as pastel.<ref name="VanessaFrance">{{Cite news |date=22 May 2011 |title=Getting the blues: the pastel trade in southwest France |language=en-US |work=Life on La Lune |url=https://vanessafrance.wordpress.com/2011/05/22/getting-the-blues-the-pastel-trade-in-southwest-france/ |access-date=23 February 2018}}</ref> For a long time, woad was the main source of blue dye in Europe. Woad was replaced by "true indigo", as trade routes opened up. Plant sources have now been largely replaced by [[synthetic dyes]].

[[Spanish explorers]] discovered an American species of indigo and began to cultivate the product in [[Guatemala]]. The English and French subsequently began to encourage indigo cultivation in their colonies in the [[West Indies]].<ref>{{cite book|last1=Pritchard|first1=James|title=In Search of Empire: The French in the Americas, 1670–1730|date=2004|publisher=Cambridge University Press|location=Cambridge|page=127}}</ref>

In North America, indigo is native to the coastal regions of the [[Southern United States|American South]] and was cultivated as early as 1622.<ref name=":1">{{Cite news |last=McKinley |first=Catherine E. |date=2011-11-07 |title=Excerpt: 'Indigo: In Search of the Color that Seduced the World' |url=https://www.npr.org/2011/11/07/142094495/excerpt-indigo-in-search-of-the-color-that-seduced-the-world |access-date=2025-08-01 |work=NPR |language=en}}</ref> [[Eliza Lucas]], whose father sent her indigo seeds as a gift, is credited with introducing the plant to the southern colonies.<ref name=":1" /><ref name=":2" /> However, it was enslaved West Africans from present-day [[Mali]], [[Guinea]], [[Ivory Coast]], [[Senegal]], and Nigeria who actually cultivated those indigo seeds and introduced indigo to North America in the form of dye,<ref name=":0" /><ref name=":1" /> where it became the colony's second-most important [[cash crop]] (after rice).<ref name=":2">{{cite web |url=http://cwh.ucsc.edu/SocialBiog.Martin.pdf |title=Eliza Lucas Pinckney:Indigo in the Atlantic World |author=Eliza Layne Martin |access-date=2013-08-24 |archive-url=https://web.archive.org/web/20100607061823/http://cwh.ucsc.edu/SocialBiog.Martin.pdf |archive-date=2010-06-07 }}</ref> Before the [[American Revolutionary War|Revolutionary War]], indigo accounted for more than one-third of the value of exports from the American colonies.<ref>[http://www.nwhm.org/Education/biography_elpinckney.html "Eliza Lucas Pinckney"] {{webarchive|url=https://web.archive.org/web/20081121015821/http://www.nwhm.org/Education/biography_elpinckney.html |date=21 November 2008 }}, ''Biographies'', National Women's History Museum, 2007, accessed 7 December 2008.</ref>

=== Isaac Newton's classification of indigo as a spectral color ===
[[File:Newton's colour circle.png|thumb|class=skin-invert-image|Indigo is one of the colors on Newton's [[color wheel]].]]

Newton introduced indigo as one of the seven base colors of his work. In the mid-1660s, when Newton bought a pair of [[Dispersive prism|prisms]] at a fair near [[Cambridge]], the [[East India Company]] had begun importing indigo dye into England,<ref>{{cite book|last=Allen|first=O.N. Allen & Ethel K.|title=The Leguminosae: a source book of characteristics, uses, and nodulation|year=1981|publisher=University of Wisconsin Press|location=Madison, Wisc.|isbn=978-0-299-08400-4|page=343|edition=null}}</ref> supplanting the homegrown [[woad]] as source of blue dye. In a pivotal experiment in the [[history of optics]], [[Isaac Newton's early life and achievements#Newton's theory of colour|the young Newton]] shone a narrow beam of sunlight through a prism to produce a rainbow-like band of colors on the wall. In describing this [[optical spectrum]], Newton acknowledged that the spectrum had a continuum of colors, but named seven: "The originall or primary colours are Red, yellow, Green, Blew, & a violet purple; together with Orang, Indico, & an indefinite varietie of intermediate gradations."<!--Use the archaic spellings (blew, orang, indico, ...) from the original text per WP:SPELLING--><ref>Newton's draft of ''[http://www.newtonproject.sussex.ac.uk/view/texts/normalized/NATP00003 A Theory Concerning Light and Colors] {{Webarchive|url=https://web.archive.org/web/20110721021035/http://www.newtonproject.sussex.ac.uk/view/texts/normalized/NATP00003 |date=21 July 2011 }}'' on newtonproject.sussex.ac.uk</ref> He linked the seven prismatic colors to the seven notes of a western [[major scale]],<ref>{{cite web |url=http://www1.umn.edu/ships/updates/newton1.htm |title=SHiPS Resource Center &#124;&#124; Newton's Colors |access-date=16 October 2010 |archive-url=https://web.archive.org/web/20140929225102/http://www1.umn.edu/ships/updates/newton1.htm |archive-date=29 September 2014 }}</ref> as shown in his color wheel, with orange and indigo as the [[semitone]]s. Having decided upon seven colors, he asked a friend to repeatedly divide up the spectrum that was projected from the prism onto the wall:
[[File:Newton prismatic colours.JPG|thumb|class=skin-invert-image|upright=1.59|Newton's observation of prismatic colors: Comparing this to a color image of the visible light spectrum shows that "blue" corresponds to [[cyan]], while "indigo" corresponds to [[blue]].]]
<blockquote>I desired a friend to draw with a pencil lines cross the image, or pillar of colours, where every one of the seven aforenamed colours was most full and brisk, and also where he judged the truest confines of them to be, whilst I held the paper so, that the said image might fall within a certain compass marked on it. And this I did, partly because my own eyes are not very critical in distinguishing colours, partly because another, to whom I had not communicated my thoughts about this matter, could have nothing but his eyes to determine his fancy in making those marks.<ref>{{cite book|last=Brewster|first=David|title=Memoirs of the life, writings and discoveries of Sir Isaac Newton, Volume 1|year=1855|page=408|url=https://books.google.com/books?id=4hY6AAAAcAAJ&q=For+some+years+past,+the+prismatic+colours+being+in+a+well+darkened+room&pg=PA408|access-date=1 November 2020|archive-date=30 June 2023|archive-url=https://web.archive.org/web/20230630145344/https://books.google.com/books?id=4hY6AAAAcAAJ&q=For+some+years+past,+the+prismatic+colours+being+in+a+well+darkened+room&pg=PA408|url-status=live}}</ref></blockquote>
[[File:Rainbow-diagram-ROYGBIV.svg|thumb|upright=0.86|The traditional seven colors of the rainbow]]
Indigo is therefore counted as one of the traditional colors of the rainbow, the order of which is given by the mnemonics "Richard of York gave battle in vain" and ''[[Roy G. Biv]]''. [[James Clerk Maxwell]] and [[Hermann von Helmholtz]] accepted indigo as an appropriate name for the color flanking violet in the spectrum.<ref name="ronchi">{{cite book|last=Ronchi|first=Lucia R.|title=The Excentric Blue. An Abridged Historical Review|year=2009|publisher=Fondazione Giorgio Ronchi|isbn=978-88-88649-19-1|url=https://books.google.com/books?id=E3rJnK0OUawC&q=Maxwell+and+von+Helmholtz+accepted+indigo&pg=PA51|author2=Jodi Sandford|access-date=1 November 2020|archive-date=30 June 2023|archive-url=https://web.archive.org/web/20230630145347/https://books.google.com/books?id=E3rJnK0OUawC&q=Maxwell%20and%20von%20Helmholtz%20accepted%20indigo&pg=PA51|url-status=live}}</ref>

Later scientists concluded that Newton named the colors differently from current usage.<ref>{{cite book|last=Evans|first=Ralph M.|title=The perception of color|year=1974|publisher=Wiley-Interscience|location=New York|isbn=978-0-471-24785-2|edition=null}}</ref><ref>{{cite journal|last=McLaren|first=K.|title=Newton's indigo|journal=Color Research & Application|date=March 2007|volume=10|issue=4|pages=225–∠229|doi=10.1002/col.5080100411}}</ref>
According to Gary Waldman, "A careful reading of Newton's work indicates that the color he called indigo, we would normally call blue; his blue is then what we would name [[blue-green]] or [[cyan]]."<ref name=Waldman2002>{{cite book|last=Waldman|first=Gary|title=Introduction to light: the physics of light, vision, and color|date=1983|page=193|edition=2002 Dover revised|publisher=Dover Publications|location=Mineola|isbn=978-0-486-42118-6|url=https://books.google.com/books?id=PbsoAXWbnr4C&q=Newton+color+Indigo&pg=PA193}}</ref> If this is true, Newton's seven spectral colors would have been:
{{center|[[Red]]:{{color box|#ff0000}} [[Orange (colour)|Orange]]:{{color box|#ff8000}} [[Yellow]]:{{color box|#ffff00}} [[Green]]:{{color box|#00ff00}} [[Blue]]:{{color box|#54baff}} Indigo:{{color box|#0000ff}} [[Violet (color)|Violet]]:{{color box|#8B00FF}}}}

The human eye does not readily differentiate hues in the wavelengths between what are now called blue and violet.{{citation needed|date=March 2025}} If this is where Newton meant indigo to lie, most individuals would have difficulty distinguishing indigo from its neighbors. According to [[Isaac Asimov]], "It is customary to list indigo as a color lying between blue and violet, but it has never seemed to me that indigo is worth the dignity of being considered a separate color. To my eyes, it seems merely deep blue."<ref>{{cite book|last=Asimov|first=Isaac|title=Eyes on the universe: a history of the telescope|year=1975|publisher=Houghton Mifflin|location=Boston|isbn=978-0-395-20716-1|page=[https://archive.org/details/eyesonuniverse00isaa/page/59 59]|url-access=registration|url=https://archive.org/details/eyesonuniverse00isaa/page/59}}</ref>

===1800s===
In 1821, [[Abraham Werner]] published ''Werner's Nomenclature of Colours'', where indigo, called ''indigo blue'', is classified as a blue hue, and not listed among the violet hues.  He writes that the color is composed of "[[Prussian blue|Berlin blue]], a little black, and a small portion of apple green," and indicating it is the color of blue [[copper ore]], with Berlin blue being described as the color of a [[blue jay]]'s wing, a [[hepatica]] flower, or a blue [[sapphire]].<ref name="Werner">{{cite book |last1=Werner |first1=Abraham |title=Werner's Nomenclature of Colours |date=1821 |location=London |page=41 |url=https://archive.org/details/gri_c00033125012743312/page/n41/mode/2up|access-date=30 October 2023}}</ref>

According to an article, ''Definition of the Color Indigo'' published in ''[[Nature (journal)|Nature]]'' magazine in the late 1800s, Newton's use of the term "indigo" referred to a spectral color between blue and violet. However, the article states that Wilhelm von Bezold, in his treatise on color, disagreed with Newton's use of the term, on the basis that the pigment indigo  was a darker hue than the spectral color; and furthermore, Professor [[Ogden Rood]] points out that indigo pigment corresponds to the cyan-blue region of the spectrum, lying between blue and green, although darker in hue. Rood considers that artificial [[ultramarine]] pigment is closer to the point of the spectrum described as "indigo", and proposed renaming that spectral point as "ultramarine". The article goes on to state that comparison of the pigments, both dry and wet, with Maxwell's discs and with the spectrum, that indigo is almost identical to [[Prussian blue]], stating that it "certainly does not lie on the violet side of 'blue.'" When scraped, a lump of indigo pigment appears more violet, and if powdered or dissolved, becomes greenish.<ref name="Nature">{{cite journal |title=Definition of the Color Indigo |journal=Littel's Living Age |date=1880 |volume=145 |issue=1869 |url=https://en.wikisource.org/wiki/Littell%27s_Living_Age/Volume_145/Issue_1869/Definition_of_the_Color_Indigo |access-date=30 October 2023 |quote=Newton denoted by the name of "indigo" the tint of the spectrum lying between "blue" and "violet." Von Bezold, in his work on color, rejects the term, justifying his objection by observing that the pigment indigo is a much darker hue than the spectrum tint. Prof. O. N. Rood, who follows Von Bezold in rejecting the term, brings forward the further objection that the tint of the pigment indigo more nearly corresponds in hue (though it is darker) with the cyan-blue region lying between green and blue. By comparing the tints of indigo pigment, both dry and wet, with the spectrum, and by means of Maxwell's disks, it appears that the hue of indigo is almost identical with that of Prussian blue, and certainly does not lie on the violet side of "blue." Indigo in the dry lump, if scraped, has, however, a more violet tint; but if fractured or powdered, or dissolved, its tint is distinctly greenish. Prof. Rood considers that artificial ultramarine corresponds much more nearly to the true tint of the spectrum at the point usually termed "indigo," and he therefore proposes to substitute the term "ultramarine" in its place, the color of the artificial pigment being thereby intended. |archive-date=30 October 2023 |archive-url=https://web.archive.org/web/20231030033534/https://en.wikisource.org/wiki/Littell%27s_Living_Age/Volume_145/Issue_1869/Definition_of_the_Color_Indigo |url-status=live }}</ref>

===Modern spectral classification===
Several modern sources place indigo in the [[electromagnetic spectrum]] between 420 and 450 nanometers,<ref name="EoF" /><ref name="Huris">{{cite web|url=http://www.huris.com/web/cog/sci/phs/phy/emr.htm|title=Spectrum of Electromagnetic Radiation ( EMR )|publisher=The HURIS Group|website=www.huris.com|access-date=14 June 2013|archive-date=28 January 2020|archive-url=https://web.archive.org/web/20200128045739/http://www.huris.com/web/cog/sci/phs/phy/emr.htm}}</ref><ref name="VIBGYOR">{{cite web|url=http://www.mathworks.com/matlabcentral/fileexchange/20718-vibgyor-color-segmentation/content/VIBGYORsegmentation.m|title=VIBGYOR Color Segmentation – File Exchange – MATLAB Central|website=www.mathworks.com|date=June 2023|access-date=14 June 2013|archive-date=19 October 2013|archive-url=https://web.archive.org/web/20131019103723/http://www.mathworks.com/matlabcentral/fileexchange/20718-vibgyor-color-segmentation/content/VIBGYORsegmentation.m|url-status=live}}</ref> which lies on the short-wave side of [[Shades of blue#Blue (RGB) (X11 blue)|color wheel (RGB) blue]], towards (spectral) violet.

The correspondence of this definition with colors of actual indigo dyes, though, is disputed. Optical scientists Hardy and Perrin list indigo as between 445<ref>{{cite web |url=https://science-edu.larc.nasa.gov/EDDOCS/Wavelengths_for_Colors.html |title=What Wavelength Goes with a Color? |access-date=24 November 2016 |archive-url=https://web.archive.org/web/20161124160412/https://science-edu.larc.nasa.gov/EDDOCS/Wavelengths_for_Colors.html |archive-date=24 November 2016 }}</ref> and 464&nbsp;nm wavelength,<ref>Arthur C. Hardy and Fred H. Perrin. ''[http://apps.isiknowledge.com/full_record.do?product=UA&search_mode=GeneralSearch&qid=22&SID=2EdCK2KejLbni4FJpgB&page=1&doc=1&colname=BIOSIS The Principles of Optics.]'' McGraw-Hill Book Co., Inc., New York. 1932.</ref> which occupies a spectrum segment from roughly the color wheel (RGB) blue extending to the long-wave side, towards [[azure (color)|azure]].

Other modern [[colorimetry|color scientists]], such as Bohren and Clothiaux (2006), and J.W.G. Hunt (1980), divide the spectrum between violet and blue at about 450&nbsp;nm, with no hue specifically named indigo.<ref name=Bohren>{{cite book | url = https://books.google.com/books?id=1oDOWr_yueIC&q=indigo+spectra+blue+violet+date:1990-2007&pg=PA214 | title = Fundamentals of Atmospheric Radiation | author = Craig F. Bohren and Eugene E. Clothiaux | publisher = Wiley-VCH | year = 2006 | isbn = 978-3-527-40503-9 }}{{Dead link|date=November 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref><ref name=hunt>{{cite book | title = Measuring Color | author = J. W. G. Hunt | year = 1980 | publisher = Ellis Horwood Ltd | isbn = 978-0-7458-0125-4}}</ref>

===Web era===
====Origin of "Indigo" as a name for purple in web pages====
Towards the end of the 20th century, purple colors also became referred to as "indigo".  In the 1980s, computer programmers [[Jim Gettys]], Paul Ravelling, John C. Thomas and Jim Fulton produced a list of colors for the [[X Window System|X Window]] Operating System. The color identified as "indigo" was not the color indigo (as generally understood at the time), but was actually a dark purple hue; the programmers assigned it the  [[Web colors|hex code]] #4B0082 {{Colorsample|#4B0082|1}}. This collection of color names was somewhat arbitrary: Thomas used a box of 72 [[Crayola]] crayons as a standard, whereas Ravelling used color swabs from the now-defunct Sinclair Paints company, resulting in the color list for version [[X11 color names#Color name chart|X11]]  of the operating system containing fanciful color names such as "papaya whip", "blanched almond" and "peach puff".  The database was also criticised for its many inconsistencies, such as "dark gray" being lighter than "gray", and for the color distribution being uneven, tending towards reds and greens at the expense of blues.<ref>{{cite news | url=https://arstechnica.com/information-technology/2015/10/tomato-versus-ff6347-the-tragicomic-history-of-css-color-names/ | title="Tomato" versus "#FF6347"—the tragicomic history of CSS color names | work=Ars Technica | date=October 11, 2015 | access-date=October 11, 2015 | first=Julianne | last=Tveten | archive-date=29 October 2023 | archive-url=https://web.archive.org/web/20231029182142/https://arstechnica.com/information-technology/2015/10/tomato-versus-ff6347-the-tragicomic-history-of-css-color-names/ | url-status=live }}</ref>

In the 1990s, this list which came with version X11 became the basis of the [[HTML]] and [[CSS]] color rendition used in websites and web design. This resulted in the name "Indigo" being associated with purple and violet hues in web page design and graphic design.  Physics author John Spacey writes on the website ''Simplicable'' that the X11 programmers did not have any background in color theory, and that as these names are used by web designers and graphic designers, the name ''indigo'' has since that time been strongly associated with purple or violet.  Spacey writes, "As such, a few programmers accidentally repurposed a color name that was known to civilisations for thousands of years."<ref name="Spacey">{{cite web |last1=Spacey |first1=John |title=19 Types of Indigo |date=19 June 2020 |url=https://simplicable.com/colors/indigo |website=Simplicable |access-date=30 October 2023 |archive-url=https://web.archive.org/web/20230605223840/https://simplicable.com/colors/indigo |archive-date=2023-06-05 |quote=In 1986 some programmers created a list of color names for a unix system known as X11. Having no background in color theory, they placed indigo as a dark purple. This list was later used by HTML and CSS standards that remain in place to this day. These standards are used by millions of designers and digital artists such that the color name indigo is now strongly associated with dark purple or violet. As such, a few programmers accidentally repurposed a color name that was known to civilizations for thousands of years. ...Note the difference between Web Indigo and Indigo. This standard color name is completely detached from the traditional color. This misrepresentation resulted from the random selection by a programmer working on an operating system in 1986. |url-status=live}}</ref>

====Crayola crayon colors====
The Crayola company released an [[Crayola#Colors chart|indigo crayon]] in 1999, with the Crayola website using the [[hex code]] #4F49C6 {{Colorsample|#4F49C6|1}} to [[List of Crayola crayon colors|approximate the crayon color]]. The 2001 iron indigo crayon is portrayed using hex code #184FA1 {{Colorsample|#184FA1|1}}. The 2004 indigo crayon color is depicted by #5D76CB {{Colorsample|#5D76CB|1}}, and the 2019 iridescent indigo is portrayed by #3C32CD {{Colorsample|#3C32CD|1}}.

== Distinction among tones of indigo ==
{{Unref section|date=March 2025}}
Like many other colors ([[Orange (colour)|orange]], [[Rose (color)|rose]], and [[violet (color)|violet]] are the best-known), indigo gets its name from an object in the natural world—the plant named [[indigo plant|indigo]] once used for dyeing cloth (see also [[Indigo dye]]).

The color pigment indigo is equivalent to the web color indigo and approximates the color indigo that is usually reproduced in pigments and colored pencils.

The color of indigo dye is a different color from either spectrum indigo or pigment indigo. This is the actual color of the dye. A vat full of this dye is a darker color, approximating the web color [[midnight blue]].

The color "electric indigo" is a bright and [[colorfulness|saturated]] color between the traditional indigo and violet. This is the [[lightness|brightest]] color indigo that can be approximated on a computer screen; it is a color located between the (primary) blue and the color [[Violet (color)#Electric violet|violet]] of the RGB color wheel.

The web color blue violet or deep indigo is a tone of indigo brighter than pigment indigo, but not as bright as electric indigo.

Listed below are several indigo hues, some of which have included the word "indigo", with the adoption of [[HTML color names]] in the [[World Wide Web]] era.

=== Indigo dye color ===
{{Main|Indigo dye}}
{{infobox color
|title=Indigo Dye
|hex=00416A
|source=<ref name="electric-coords" />
|isccname=Deep blue}}
''Indigo dye'' is a greenish dark blue color, obtained from either the leaves of the tropical Indigo plant (''[[Indigofera]]''), or from woad (''[[Isatis tinctoria]]''), or the Chinese indigo (''[[Persicaria tinctoria]]''). Many societies make use of the ''[[Indigofera]]'' plant for producing different shades of blue. Cloth that is repeatedly boiled in an indigo dye bath-solution (boiled and left to dry, boiled and left to dry, etc.), the blue pigment becomes darker on the cloth. After dyeing, the cloth is hung in the open air to dry.

A Native American woman described the process used by the [[Cherokee Indians]] when extracting the dye:
<blockquote>We raised our indigo which we cut in the morning while the dew was still on it; then we put it in a tub and soaked it overnight, and the next day we foamed it up by beating it with a gourd. We let it stand overnight again, and the next day rubbed [[tallow]] on our hands to kill the foam. Afterwards, we poured the water off, and the sediment left in the bottom we would pour into a pitcher or crock to let it get dry, and then we would put it into a poke made of cloth (i.e. sack made of coarse cloth) and then when we wanted any of it to dye [there]with, we would take the dry indigo.<ref>{{Citation |contribution=History of the Cherokees, 1830–1846 |title=Chronicles of Oklahoma |last1=Knight |first1=Oliver |publisher=Oklahoma Historical Society|place=Oklahoma City|year=1956–57|page=164|oclc=647927893 |language=en }}</ref><ref>{{cite book|author-last=Foreman|author-first=Grant|title=The Five Civilized Tribes|publisher=University of Oklahoma Press|location=Norman|year=1934|page=283|language=en|url=https://books.google.com/books?id=gGtLnSkqkekC&q=We+raised+our+indigo&pg=PA283|isbn=978-0-8061-0923-7|access-date=1 November 2020|archive-date=30 June 2023|archive-url=https://web.archive.org/web/20230630145344/https://books.google.com/books?id=gGtLnSkqkekC&q=We+raised+our+indigo&pg=PA283|url-status=live}}</ref></blockquote>

In [[Sa Pa]], Vietnam, the tropical Indigo (''Indigo tinctoria'') leaves are harvested and, while still fresh, placed inside a tub of room-temperature to lukewarm water where they are left to sit for 3 to 4 days and allowed to ferment, until the water turns green. Afterwards, crushed limestone ([[pickling lime]]) is added to the water, at which time the water with the leaves are vigorously agitated for 15 to 20 minutes, until the water turns blue. The blue pigment settles as sediment at the bottom of the tub. The sediment is scooped out and stored. When dyeing cloth, the pigment is then boiled in a vat of water; the cloth (usually made from yarns of [[hemp]]) is inserted into the vat for absorbing the dye. After hanging out to dry, the boiling process is repeated as often as needed to produce a darker color.
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=== Indigo (color wheel) ===
{{infobox color
| title=Indigo (color wheel)
| hex=4000FF
| hexref=<ref name="electric-coords">{{cite web |title=What Color is Indigo? |work=About.com Tech |url=http://desktoppub.about.com/od/choosingcolors/f/What-Color-Is-Indigo.htm |date=16 December 2014 |archive-url=https://web.archive.org/web/20151226130706/http://desktoppub.about.com/od/choosingcolors/f/What-Color-Is-Indigo.htm |archive-date=26 December 2015}}</ref>
|isccname=Vivid purplish blue}}

In a [[RGB color space]], "Indigo(color wheel)" is composed of 25.1% [[red]], 0% [[green]] and 100% [[blue]]. Whereas in a CMYK color space, it is composed of 74.9% [[cyan]], 100% [[magenta]], 0% [[yellow]] and 0% [[black]]. It has a hue angle of 255.1 degrees, a saturation of 100% and a lightness of 50%. Indigo(color wheel) could be obtained by blending [[Violet (color)#Violet(color wheel)|violet]] with [[blue]].
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=== Electric indigo ===
{{infobox color
| title=Electric Indigo
| hex=6F00FF
| hexref=<ref name="electric-coords"/>
|isccname=Vivid purplish blue}}

"Electric indigo" is brighter than the pigment indigo reproduced above. When plotted on the [[CIE chromaticity diagram]], this color is at 435 nanometers, in the middle of the portion of the spectrum traditionally considered indigo, i.e., between 450 and 420 nanometers. This color is only an approximation of spectral indigo, since actual spectral colors are outside the [[gamut]] of the [[sRGB]] color system.
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=== Deep indigo (web color blue-violet) ===
{{infobox color
|title=Blue-Violet
|hex=8A2BE2
|source=[[Web colors#X11 color names|X11]]
|isccname=Vivid violet}}

At right is displayed the web color "blue-violet", a color intermediate in brightness between electric indigo and pigment indigo. It is also known as "deep indigo".
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=== Web color indigo ===
{{infobox color
|title=Web color Indigo
|hex=4B0082
|source=[[colorhexa.com]]<ref>{{Cite web|url=https://www.colorhexa.com/4b0082|title=Indigo / #4b0082 hex color|website=ColorHexa}}</ref>
|isccname=Dark violet}}

{{infobox color
|title=Web safe Indigo
|hex=330099
|source=[[rgbcolorcode.com]]<ref>{{Cite web|url=https://rgbcolorcode.com/color/330099|title=#330099 🎨 RGB Color Code|website=rgbcolorcode.com}}</ref>
|isccname=}}

The color box on the right displays the web color indigo. Its hue is closer to violet than to indigo dye for which the color is named.
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=== Tropical indigo ===
{{infobox color
|title=Tropical Indigo
|hex=9683EC
|source=Gallego and Sanz<ref>Gallego, Rosa; Sanz, Juan Carlos (2005). ''Guía de coloraciones'' (Gallego, Rosa; Sanz, Juan Carlos (2005). ''Guide to Colorations'') Madrid: H. Blume. {{ISBN|84-89840-31-8}}</ref>
|isccname=Vivid violet}}

'Tropical Indigo' is the color that is called ''añil'' in the ''Guía de coloraciones'' (Guide to colorations) by Rosa Gallego and
Juan Carlos Sanz, a color dictionary published in 2005 that is widely popular{{According to whom?|date=March 2025}} in the [[Hispanophone]] realm.
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===Imperial blue===
{{infobox color
| title=Imperial Blue
| hex=002395
| isccname=Vivid blue}}
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== In nature ==
===Birds===
* Male [[indigobird]]s are a very dark, metallic blue.
* The [[indigo bunting]], native to North America, is mostly bright [[cerulean]] blue with an indigo head.
* The related [[blue grosbeak]] is, ironically, more indigo than the indigo bunting.

===Fungi===
* ''[[Lactarius indigo]]'' is one of the very few species of [[mushroom]]s colored in tones of blue.

===Snakes===
* The [[eastern indigo snake]], ''Drymarchon couperi'', of the southeastern United States, is a dark blue/black.

<gallery class="center" widths="180px" heights="180px" >
File:Indigo Bunting.jpg|[[Indigo bunting]]
File:Lactarius indigo 48568 edit.jpg|An upturned ''[[Lactarius indigo]]'' mushroom
File:Eastern Indigo Snake.jpg|[[Eastern indigo snake]]
</gallery>

== In culture ==
=== Business ===
* [[IndiGo]] is an Indian budget airline that uses an indigo logo and operates only [[Airbus A320]]s.
* [[Indigo Books and Music]] uses an indigo logo and has sometimes referred to the color as "blue" in advertising.<ref>"It's New and It's Blue" (Indigo advertisement), ''[[The Globe and Mail]]'', Toronto, 1 October 1999, p.&nbsp;A3</ref><ref>"Indigo Bookstore had a 'Think Blue' campaign back in 1999" according to: {{cite web|title=Think Blue 2008: a Before and After Tale of Silly Turf Battles and Redemptive Communication|url=http://www.irregulartimes.com/thinkblue2008.html|access-date=4 February 2013|archive-date=10 May 2012|archive-url=https://web.archive.org/web/20120510202417/http://www.irregulartimes.com/thinkblue2008.html|url-status=usurped}}{{better source needed|date=February 2013}}</ref>
* The [[GameCube]] was initially released in two color variants. One bore the title of 'Indigo', with the main console and controllers in that color.

=== Computer graphics ===
* Electric indigo is sometimes used as a glow color for [[computer graphics lighting]], possibly because it seems to change color from indigo to [[lavender (color)|lavender]] when blended with white.

=== Dyes ===
{{further|Indigo dye#History}}
[[File:COLLECTIE TROPENMUSEUM Indigo wordt aangemaakt in kuilen in uitgehakte puimsteentufgrond Karolanden TMnr 10014190.jpg|thumb|Indigo is created in potholes carved in pumice "tufgrond" in [[Karo Regency|Karoland]], [[Sumatra]].]]
* Indigo dye was used to dye denim, giving the original '[[Jeans|blue jeans]]' their distinctive colour.
* The original Postal Worker uniform contained indigo dye, partly due to the dye not running when wet.<ref>{{cite web |last1=Hejna |first1=Jamie |title=What Color is Indigo? |url=https://freelancefaucet.com/answered/what-color-is-indigo/ |website=Freelance Faucet |date=19 February 2023 |access-date=19 February 2023 |archive-date=19 February 2023 |archive-url=https://web.archive.org/web/20230219153939/https://freelancefaucet.com/answered/what-color-is-indigo/ |url-status=live }}</ref>
* [[Guatemala]], as of 1778, was considered one of the world's foremost providers of indigo.<ref name="Kitchin">{{cite book|last=Kitchin|first=Thomas|title=The Present State of the West-Indies: Containing an Accurate Description of What Parts Are Possessed by the Several Powers in Europe|year=1778|publisher=R. Baldwin|location=London|page=30|url=http://www.wdl.org/en/item/4397/view/1/30/|access-date=30 August 2013|archive-date=19 October 2013|archive-url=https://web.archive.org/web/20131019122932/http://www.wdl.org/en/item/4397/view/1/30/|url-status=live}}</ref>
* In Mexico, indigo is known as ''[[Indigofera suffruticosa|añil]]''.<ref>Gallego, Rosa; Sanz, Juan Carlos (2001). Diccionario Akal del color. Akal. {{ISBN|978-84-460-1083-8}}.</ref> After silver, and [[cochineal]] to produce red, ''añil'' was the most important product exported by historical Mexico.<ref>Article "añil" in: ''Enciclopedia de México'', vol 1, Mexiko-City: Secretaría de Educacion Pública, 1987</ref>
* The use of ''añil'' is survived in the Philippines, particularly in the Visayas and Mindanao. The powder dye is mixed with vinegar to be applied to the cheek of a person suffering from mumps.<ref>{{Cite web |last=Ph.D |first=DR ANTHONY MELVIN CRASTO |date=2021-04-01 |title=Indigo |url=https://www.allfordrugs.com/2021/04/01/indigo/ |access-date=2025-07-13 |website=All About Drugs |language=en-US}}</ref>

=== Food ===
* Scientists discovered in 2008 that when a [[banana]] becomes ripe, it glows bright indigo under a [[black light]]. Some insects, as well as birds, see into the [[ultraviolet]], because they are [[Tetrachromacy|tetrachromats]] and can use this information to tell when a banana is ready to eat. The glow is the result of a chemical created as the green [[chlorophyll]] in the peel breaks down.<ref>{{cite magazine |first= Rachel |last= Zurer |title= Three Smart Things About Banana Peels |url= https://www.wired.com/2011/09/st_3st_banana/ |access-date= 28 February 2020 |magazine= [[Wired (magazine)|Wired]] |df= mdy-all |archive-date= 28 February 2020 |archive-url= https://web.archive.org/web/20200228213507/https://www.wired.com/2011/09/st_3st_banana/ |url-status= live }}</ref>

=== Literature ===
Marina Warner's novel ''[[Indigo (Warner novel)|Indigo]]'' (1992) is a retelling of Shakespeare's ''[[The Tempest]]'' and features the production of indigo dye by Sycorax.

=== Military ===
The [[French Army]] adopted dark blue indigo at the time of the [[French Revolution]], as a replacement for the white uniforms previously worn by the Royal infantry regiments. In 1806, [[Napoleon]] decided to restore the white coats because of shortages of [[indigo dye]] imposed by the British continental blockade. However, the greater practicability of the blue color led to its retention, and indigo remained the dominant color of French military coats until 1914.<ref>{{Cite web |last=Ph.D |first=DR ANTHONY MELVIN CRASTO |date=2021-04-01 |title=Indigo |url=https://www.allfordrugs.com/2021/04/01/indigo/ |access-date=2025-07-13 |website=All About Drugs |language=en-US}}</ref>

=== Spirituality ===
The [[Spiritualism (movement)|spiritualist]] applications use [[#Electric indigo|electric indigo]], because the color is positioned between [[blue]] and [[Shades of violet#Electric violet|violet]] on the spectrum.<ref>Tansley, David W. ''Subtle Body: Essence and Shadow'' 1984 (Art and Cosmos Series--[[Jill Purce]], editor)</ref>
* In West and Central Africa, indigo has been cultivated for millennia and plays a significant roles in the traditional spiritual beliefs of the [[Bamileke people|Bamileke]], [[Yoruba people|Yoruba]], [[Baoulé people|Baoulé]], [[Dogon people|Dogon]], [[Fon people|Fon]], and [[Aja people|Aja]] peoples.<ref name=":0" />
* The color electric indigo is used in [[New Age]] philosophy to symbolically represent the sixth ''[[chakra]]'' (called ''[[Ajna]]''), which is said to include the [[third eye]]. This ''chakra'' is believed to be related to [[intuition]] and [[gnosis]] (spiritual knowledge).<ref>Stevens, Samantha. The Seven Rays: a Universal Guide to the Archangels. City: Insomniac Press, 2004. {{ISBN|1-894663-49-7}} pg. 24</ref><ref>Graham, Lanier F. (editor) ''The Rainbow Book'' Berkeley, California:1976 Shambala Publishing and The Fine Arts Museums of San Francisco (Handbook for the Summer 1976 exhibition ''The Rainbow Art Show'' which took place primarily at the [[De Young Museum]], but also at other museums) ''Indigo'' Pages 152–153 The color indigo is stated to represent intuition.</ref>
* [[Alice Bailey|Alice A. Bailey]] used indigo as the "second ray", representing "Love-Wisdom", in her [[Seven Rays]] system classifying people into seven metaphysical [[psychological type]]s.<ref>{{cite book|last=Bailey|first=Alice A.|author-link=Alice Bailey|title=The Seven Rays of Life|location=New York|year=1995|publisher=Lucis Publishing Company|isbn=978-0-85330-142-4}}</ref>
* [[Psychic]]s often associate indigo [[Aura (paranormal)|paranormal auras]] with an interest in [[religion]] or with intense [[spirituality]] and intuition. [[Indigo children]] are said to have predominantly indigo auras. People with indigo auras are said to favor occupations such as [[Computer analyst (disambiguation)|computer analyst]]<!-- dab link intentional -->, [[Animal husbandry|animal caretaker]], and [[Licensed professional counselor|counselor]].<ref>Oslie, Pamalie ''Life Colors: What the Colors in Your Aura Reveal'' Novato, California:2000--New World Library Indigo Auras: Pages 161–174</ref>
* In [[Wicca]], it represents emotion, fluidity, insight, and expressiveness. It is used to spiritually heal.<ref>{{Cite web|title=Magical Properties of Colors|url=https://wiccaliving.com/magical-properties-colors/|access-date=2021-01-28|website=Wicca Living|language=en-US|archive-date=22 January 2021|archive-url=https://web.archive.org/web/20210122023531/https://wiccaliving.com/magical-properties-colors/|url-status=live}}</ref>

== See also ==
* ''[[Baptisia]]'' (false indigo), a genus of flowering plants
* [[Indigo dye]], used in dyeing blue jeans their characteristic color
* ''[[Indigofera]]'', a genus of flowering plants
* [[Lists of colors]]
* ''[[Persicaria tinctoria]]'', Japanese Indigo
* ''[[Red, White, and Black Make Blue: Indigo in the Fabric of Colonial South Carolina Life]]'' by [[Andrea Feeser]]

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== References ==
{{Reflist}}

== External links ==
* [https://archive.org/details/gri_c00033125012743312/page/n41/mode/2up 'Blues'] from ''Werner's Nomenclature of Colours'' (Edinburgh: William Blackwood, 1821) showing "Indigo" alongside other shades of blue
* {{Wikisource inline|Littell's Living Age/Volume 145/Issue 1869/Definition of the Color Indigo|An 1869 debate over whether "indigo" is an appropriate term for the spectral color.}}

{{Color topics}}
{{Shades of indigo}}
{{Shades of blue|Indigo}}
{{Shades of violet}}
{{Shades of lavender|Tropical Indigo}}
{{Electromagnetic spectrum}}

[[Category:Tertiary colors]]
[[Category:Quaternary colors]]
[[Category:Optical spectrum]]
[[Category:Rainbow colors]]
[[Category:Shades of blue]]