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.

Going back to BLUE and TURQUOISE: I often see a peacock colour used as "blue" in contexts that make me think there are people who see that as a perfect shade of BLUE:





Again, you might call this BLUE when presented in contrast with YELLOW, RED, and GREEN:





I wonder if that is "blue" to people who don't think of TURQUOISE as a separate colour? It would be logical that those who distinguish the two shift the locus of perfect BLUE so that is further away from any hint of TURQUOISE for maximal differentiation.





The truth is that each basic colour term applies to a wide range of actual shades; but, in the real world, we mostly don't see them in isolation. Instead, they are seen in a context where they're contrasted with the other colours around them.

In a garden, there are very few flowers that are any shade of BLUE. Delphiniums are one, though they also come in other colours. Blue lobelias and iris are at least a shade of indigo. However, the word "blue" is often used to describe the bluest-flowering varieties of such plants as petunias, which come in shades of magenta, plum, and pink. So-called "blue" petunias are actually PURPLE. (It's the magenta petunias that are called "purple".)







This photo of a delphinium comes from Wikimedia Commons, to which it was uploaded by Harry Chen on 14 January 2011.

This photo of Lobelia erinus comes from Wikimedia Commons, to which it was uploaded by Bernt Fransson on 1 August 2015.

This photo of "Easy Wave Blue" petunias comes from Wikimedia Commons, to which it was uploaded by David J. Stang on 11 August 2006.