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.

Indigofera tinctoria and related species were cultivated in 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.

[7] The Ancient Greek term for the dye was Ἰνδικὸν φάρμακον (indikon pharmakon, "Indian dye"), which, adopted to Latin as indicum (a second declension noun) or indico (oblique case) and via Portuguese, gave rise to the modern word indigo.

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.

In a pivotal experiment in the history of optics, 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.

Having decided upon seven colors, he asked a friend to repeatedly divide up the spectrum that was projected from the prism onto the wall: 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.

[16]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.

"[20] If this is true, Newton's seven spectral colors would have been: The human eye does not readily differentiate hues in the wavelengths between what are now called 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.

[23] Several modern sources place indigo in the electromagnetic spectrum between 420 and 450 nanometers,[1][24][25] which lies on the short-wave side of color wheel (RGB) blue, towards (spectral) violet.

Optical scientists Hardy and Perrin list indigo as between 445[26] and 464 nm wavelength,[27] which occupies a spectrum segment from roughly the color wheel (RGB) blue extending to the long-wave side, towards azure.

Hunt (1980), divide the spectrum between violet and blue at about 450 nm, with no hue specifically named indigo.

[30] 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.

Spacey writes, "As such, a few programmers accidentally repurposed a color name that was known to civilisations for thousands of years.

A Native American woman described the process used by the Cherokee Indians when extracting the dye: 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.

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.

[33][34]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.

In 1806, Napoleon decided to restore the white coats because of shortages of indigo dye imposed by the British continental blockade.