What is the color indigo? On the spectrum as we see it, the area made up of colors that look more or less blue is quite broad. Shades known as "indigo" are darker and in some cases verge on purple. But there's no hard boundary.
There's an interesting backstory. Isaac Newton codified the spectrum as it's known to modern people. He wanted seven colors because he had a surprising yen for numerology, and indigo dye was a well-known reference point for his contemporaries. And there are various reasons to maintain the concept of indigo. Not least because otherwise Roy G. Biv loses a needed vowel.
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Most humans have three cones in our eyes that allow us to discern the colors of the spectrum. There are a number of other colors that are non-spectral, those that just happen to be similar enough to spectral colors that people call them by the same name. Mixing wavelengths isn't the same as mixing paint. Our eyes perceive different wavelengths of light as different colors, but that doesn't mean every color we perceive has its own wavelength.
It's interesting that although indigo is between blue and purple on the color wheel orange is between red and yellow on the wheel but there is a clear point where the color is no longer yellow or red and is distinctly orange. I feel like that point never happens with indigo and every indigo color could also be described as simply navy blue or purple.
It's true Newton had a surprising yen for numerology - in fact his book Opticks is considered to be one of the great works in the history of science.
https://en.wikipedia.org/wiki/Opticks
Did you know that tigers are bright orange because their prey can't differentiate between green and orange?
Right. It's a matter of different wavelengths causing interactions between the various cones. Which is why there's a subjective element to color even before you get to conditions like selective colorblindness. Seeing non-spectral colors as well as spectral ones likely conferred a survival benefit to out ancestors. It still does, but there's also the fact that it makes out world more beautiful. We're lucky to see what we do.
I agree that orange has its own quality in a way that indigo doesn't, really. Yes, it certainly has a resemblance to red on one side and yellow on the other. But as long as the language you're speaking has a word for orange, you'll be able to identify it as such. Indigo is more a fuzzy border between two colors.
Considering the time, it's interesting that Opticks was first published in English. I believe at the time that it was standard to publish scholarly works in Latin first and foremost. Newton must have had things he just had to say now.
Yeah, dichromacy. It would really mess us up in that environment. Their prey probably hears them before seeing them.
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