Main Difference
The main difference between Blue and Indigo is that the Blue is a color; additive and subtractive (RYB) primary color; visible between purple and green and Indigo is a deep and bright shade of blue.
Blue
Blue is one of the three primary colours of pigments in painting and traditional colour theory, as well as in the RGB colour model. It lies between violet and green on the spectrum of visible light. The eye perceives blue when observing light with a dominant wavelength between approximately 450 and 495 nanometres. Most blues contain a slight mixture of other colors; azure contains some green, while ultramarine contains some violet. The clear daytime sky and the deep sea appear blue because of an optical effect known as Rayleigh scattering. An optical effect called Tyndall scattering explains blue eyes. Distant objects appear more blue because of another optical effect called atmospheric perspective.
Blue has been an important colour in art and decoration since ancient times. The semi-precious stone lapis lazuli was used in ancient Egypt for jewellery and ornament and later, in the Renaissance, to make the pigment ultramarine, the most expensive of all pigments. In the eighth century Chinese artists used cobalt blue to colour fine blue and white porcelain. In the Middle Ages, European artists used it in the windows of Cathedrals. Europeans wore clothing coloured with the vegetable dye woad until it was replaced by the finer indigo from America. In the 19th century, synthetic blue dyes and pigments gradually replaced mineral pigments and synthetic dyes. Dark blue became a common colour for military uniforms and later, in the late 20th century, for business suits. Because blue has commonly been associated with harmony, it was chosen as the colour of the flags of the United Nations and the European Union.Surveys in the US and Europe show that blue is the colour most commonly associated with harmony, faithfulness, confidence, distance, infinity, the imagination, cold, and sometimes with sadness. In US and European public opinion polls it is the most popular colour, chosen by almost half of both men and women as their favourite colour. The same surveys also showed that blue was the colour most associated with the masculine, just ahead of black, and was also the colour most associated with intelligence, knowledge, calm and concentration.
Indigo
Indigo is a deep and rich 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 violet and blue; however, sources differ as to its actual position in the electromagnetic spectrum.
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.
Blue (adjective)
Of the colour blue.
“the deep blue sea”
Blue (adjective)
Depressed, melancholic, sad.
Blue (adjective)
Pale, without redness or glare; said of a flame.
“The candle burns blue.”
Blue (adjective)
Supportive of, run by (a member of), pertaining to, or dominated by a political party represented by the colour blue.
Blue (adjective)
Supportive of, run by (a member of), pertaining to, or dominated by the Democratic Party. after 2000
“I live in a blue constituency.”
“Congress turned blue in the mid-term elections.”
Blue (adjective)
Of the higher-frequency region of the part of the electromagnetic spectrum which is relevant in the specific observation.
Blue (adjective)
Extra rare; left very raw and cold.
Blue (adjective)
Having a coat of fur of a slaty gray shade.
Blue (adjective)
Severe or overly strict in morals; gloomy.
“blue and sour religionists;”
“blue laws”
Blue (adjective)
literary; bluestockinged.
Blue (adjective)
Having a color charge of blue.
Blue (adjective)
Risque or obscene
“His material is too blue for prime-time”
Blue (adjective)
Pornographic or profane.
“The air was blue with oaths.”
“a blue movie”
Blue (noun)
The colour of the clear sky or the deep sea, between green and violet in the visible spectrum, and one of the primary additive colours for transmitted light; the colour obtained by subtracting red and green from white light using magenta and cyan filters; or any colour resembling this.
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Blue (noun)
A blue dye or pigment.
Blue (noun)
Any of several processes to protect metal against rust.
Blue (noun)
Blue clothing
“The boys in blue marched to the pipers.”
Blue (noun)
A blue uniform. See blues.
Blue (noun)
A member of law enforcement
Blue (noun)
The sky, literally or figuratively.
“The ball came out of the blue and cracked his windshield.”
“His request for leave came out of the blue.”
Blue (noun)
The ocean; deep waters.
Blue (noun)
Anything blue, especially to distinguish it from similar objects differing only in color.
Blue (noun)
A dog or cat with a slaty gray coat.
Blue (noun)
One of the colour balls used in snooker, with a value of five points.
Blue (noun)
Any of the butterflies of the subfamily noshow=1 in the family Lycaenidae, most of which have blue on their wings.
Blue (noun)
A bluefish.
Blue (noun)
An argument.
Blue (noun)
A liquid with an intense blue colour, added to a laundry wash to prevent yellowing of white clothes.
Blue (noun)
A type of firecracker.
Blue (noun)
A pedantic woman; a bluestocking.
Blue (noun)
One of the three color charges for quarks.
Blue (verb)
To make or become blue.
Blue (verb)
To treat the surface of steel so that it is passivated chemically and becomes more resistant to rust.
Blue (verb)
To brighten by treating with blue (laundry aid)
Blue (verb)
To spend (money) extravagantly; to blow.
Indigo (noun)
A purplish-blue colour
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Indigo (noun)
An indigo-colored dye obtained from certain plants (the indigo plant or woad), or a similar synthetic dye.
Indigo (noun)
An indigo plant, such as from species in genera Indigofera, Amorpha (false indigo), Baptisia (wild indigo), and ver=161009 and Dalea (indigo bush).
Indigo (adjective)
having a deep purplish-blue colour