|Wavelength 450–420(disputed) nm|
sRGB (r, g, b) (75, 0, 130)
HSV (h, s, v) (275°, 100%, 51%)
|Hex triplet #4B0082|
CMYK (c, m, y, k) (42, 100, 0, 49)
Indigo is a deep and bright color close to the color wheel blue (a primary color in the RGB color space), as well as to some variants of ultramarine. It is traditionally regarded as a color in the visible spectrum, as well as one of the seven colors of the rainbow: the color between blue and violet; however, sources differ as to its actual position in the electromagnetic spectrum.
- Classification as a spectral color
- Distinction between the four major tones of indigo
- Electric indigo
- Deep indigo (web color blue violet)
- Light indigo (web color indigo)
- Tropical indigo
- In nature
- Computer graphics
The color indigo is named after the indigo dye derived from the plant Indigofera tinctoria and related species.
The first known recorded use of indigo as a color name in English was in 1289.
Species of Indigofera were cultivated in Peru, India, East Asia and Egypt in antiquity. The earliest direct evidence for the use of indigo dates to around 4000 BCE and comes from Huaca Prieta, in contemporary Peru. Pliny mentions India as the source of the dye, imported in small quantities via the Silk Road. The Greek term for the dye was Ἰνδικὸν φάρμακον ("Indian dye"), which, adopted to Latin as indicum and via Portuguese gave rise to the modern word indigo. El Salvador has lately been the biggest producer of indigo.
Indigo was actually a plant that got its name because it came from the Indus Valley, discovered some 5,000 years ago, where it was called nila, meaning dark blue. And by the 7th Century BC, people starting using the plant as a dye — the Mesopotamians were even carving out recipes for making indigo dye onto clay tablets for record-keeping. By 1289, knowledge of the dye made its way to Europe, when the Venetian merchant traveler Marco Polo reported on it.
But it wasn’t until 1640 when demand started to pick up for 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. Indigo dye could be made from two different types of plants — the indigo plant, which produced the best results, and from the woad plant. The British were producing indigo with woad, a plant that yielded a lesser quality dye, but a plant they could grow. They even tried to hold their monopoly on indigo dye by managing to ban the indigo plant for years, claiming that it was poisonous. But eventually the British began to focus on tea and other crops — and meanwhile, the French started to get their fair share of the market. But this was problematic. The French had gone to war with Britain, so the British could hardly rely on the French for this precious blue dye. Consequently, the British had to turn to their colonies in America. It was Eliza Lucas from South Carolina who figured out how to grow the indigo plant and use it to make indigo cakes that would support British demand.
The same indigo dye is contained in the woad plant, Isatis tinctoria, for a long time the main source of blue dye in Europe. Woad was replaced by true indigo as trade routes opened up, and both are now largely replaced by synthetic dyes.
The Early Modern English word indigo referred to the dye, and not to the color (hue) itself, and indigo is not traditionally part of the basic color-naming system.
Classification as a spectral color
Modern sources place indigo in the spectrum between 420 and 450 nanometers, which lies on the short-wave side of color wheel (RGB) blue, towards (spectral) violet. However, the correspondence of this definition with colors of actual indigo dyes is disputed. Optical scientists Hardy and Perrin list indigo as between 445 and 464 nm wavelength, which occupies a spectrum segment from roughly the color wheel (RGB) blue extending to the long-wave side, towards azure.
Isaac Newton introduced indigo as one of the seven base colors of his work. In the mid-1660s, when Newton bought a pair of prisms at a fair near Cambridge, the East India Company had begun importing indigo dye into England, supplanting the homegrown woad as source of blue dye. 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." He linked the seven prismatic colors to the seven notes of a western major scale, as shown in his color wheel, with orange and indigo as the semitones. 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. 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.
Indigo is therefore counted as one of the traditional colors of the rainbow, the order of which is given by the mnemonic Roy G. Biv. James Clerk Maxwell and Hermann von Helmholtz accepted indigo as an appropriate name for the color flanking violet in the spectrum.
Later scientists conclude that Newton named the colors differently from current usage. 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, cyan or light blue." If this is true, Newton's seven spectral colors would have been:
The human eye does not readily differentiate hues in the wavelengths between blue and violet.‹See TfD› 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."
Modern color scientists typically divide the spectrum between violet and blue at about 450 nm, with no indigo.
Distinction between the four major tones of indigo
Like many other colors (orange, rose, and violet are the best-known), indigo gets its name from an object in the natural world—the plant named indigo once used for dyeing cloth (see also Indigo dye).
The color electric indigo is a bright and saturated color between the traditional indigo and violet. This is the brightest color indigo that can be approximated on a computer screen—it is a color located between the (primary) blue and the color 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.
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.
Below are displayed these four major tones of indigo.
The color 'electric indigo' is much brighter than the pigment indigo reproduced below. 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.
Deep indigo (web color blue-violet)
At right is displayed the web color 'blue-violet', a color intermediate in brightness between electric indigo and pigment indigo. This color is also called 'deep indigo'.
Light indigo (web color indigo)
The color box at right displays the web color indigo which is equivalent to 'light indigo', the color indigo as it would be reproduced by artists' paints as opposed to the brighter indigo above (electric indigo) that is possible to reproduce on a computer screen. Its hue is closer to violet than to indigo dye for which the color is named. Pigment indigo can be obtained by mixing 55% pigment cyan with about 45% pigment magenta.
Compare the subtractive colors to the additive colors in the two primary color charts in the article on primary colors to see the distinction between electric colors as reproducible from light on a computer screen (additive colors) and the pigment colors reproducible with pigments (subtractive colors); the additive colors are significantly brighter because they are produced from light instead of pigment.
Light indigo (web color indigo) represents the way the color indigo was always reproduced in pigments, paints, or colored pencils in the 1950s. By the 1970s, because of the advent of psychedelic art, artists became used to brighter pigments, and pigments called "bright indigo" or "bright blue-violet" that are the pigment equivalent of the electric indigo reproduced in the section above became available in artists' pigments and colored pencils.
'Tropical Indigo' is the color that is called añil (the Spanish word for "tropical indigo") 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 in the Hispanophone realm.
The tone of indigo used in the spiritualist applications is electric indigo because the color is represented as being the color of the spectrum between blue and violet.
Indigo in its dark blue shade was adopted by the French Army at the time of the 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.