Given that I've already done TURQUOISE (Newton's "blue") and BLUE (Newton's "indigo"), what's left for INDIGO?

Well, the plant produces a blue-purple dye; and the colour is usually described as lying between blue and purple. So what's that?





Some might call this simply BLUE, especially if it presented in contrast with YELLOW, RED, and GREEN:





If, however, we compare it with BLUE and PURPLE (what I think of a a perfect PURPLE), then we can see it lies in between:





What we're looking at here is, I think, a sort of transitional colour: one that lies on the border between two basic colours. In the same way, GOLD is transitional between ORANGE and YELLOW; and CHARTREUSE is transitional between GREEN (especially lime green) and YELLOW.
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