> Newton divided his color circle, which he constructed to explain additive color mixing, into seven colors. His color sequence including the tertiary color indigo is kept alive today by the Roy G. Biv mnemonic. Originally he used only five colors, but later he added orange and indigo to match the number of musical notes in the major scale.
So, the concept of there being uniquely-important indigo (and orange!) points on the hue spectrum were just, kinda... made up, as a bit of numerological whimsy, by Isaac Newton. And everyone else in the Western world just followed his lead, because he seemed to know what he was talking about.
I would note that no English-speaker actually seems to describe anything as “indigo” in practice. People do tend to describe plenty of things as “light blue”, though! We get taught “ROY G BIV” in kindergarten, but we basically ignore it, because it doesn’t actually fit anything else we get taught. (I don’t even recall any children’s picture books that bother with examples of “indigo” objects.)
IMHO, there’s also a much more fundamental distinction made in English-in-practice, between three kinds of green: yellow-green (“spring green”, “olive green” when desaturated), “green green”†, and blue-green (“sea blue”, cyan, turquoise.) Many people will insist that, if orange is a separate color from yellow or red, then yellow-green and green-blue are separate colors as well.
> Newton divided his color circle, which he constructed to explain additive color mixing, into seven colors. His color sequence including the tertiary color indigo is kept alive today by the Roy G. Biv mnemonic. Originally he used only five colors, but later he added orange and indigo to match the number of musical notes in the major scale.
So, the concept of there being uniquely-important indigo (and orange!) points on the hue spectrum were just, kinda... made up, as a bit of numerological whimsy, by Isaac Newton. And everyone else in the Western world just followed his lead, because he seemed to know what he was talking about.
I would note that no English-speaker actually seems to describe anything as “indigo” in practice. People do tend to describe plenty of things as “light blue”, though! We get taught “ROY G BIV” in kindergarten, but we basically ignore it, because it doesn’t actually fit anything else we get taught. (I don’t even recall any children’s picture books that bother with examples of “indigo” objects.)
IMHO, there’s also a much more fundamental distinction made in English-in-practice, between three kinds of green: yellow-green (“spring green”, “olive green” when desaturated), “green green”†, and blue-green (“sea blue”, cyan, turquoise.) Many people will insist that, if orange is a separate color from yellow or red, then yellow-green and green-blue are separate colors as well.
† https://en.wikipedia.org/wiki/Contrastive_focus_reduplicatio...