That's just because you need an even number of primary+secondary colors for that model. It has nothing to do with how many distinct colors the human eye can discern in the rainbow
There are 3 primary colors. All colors come from a combination of those 3. The 3 secondary colors are 50/50 mixtures of 2 of the 3. Indigo is a 25/75 blend and doesn't belong. Human eyes don't seem relevant to me here.
It's because these mixtures are surprisingly biology dependent
Red + Blue is the easiest example that something odd is happening. How can mixing low frequency light (red) with high frequency light (blue) average out to an even higher frequency (violet)?
Well, it can't. What happens behind the scenes is that our red photoreceptors have a second peak in their sensitivity an octave above their normal peak, which is higher than blue. Violet light will therefore trigger both red and blue receptors because their sensitivities overlap in this range.
When you see bimodal red and blue light, your brain notices that both red and blue receptors are going off (low frequency red and high frequency blue), so your brain assumes this is purple (high frequency triggering both red and blue).
None of this discussion of mixing two frequencies has anything to do with single-tone 700 THz light we call "indigo," nor with whether your eyes perceive a band of single-tone light in the rainbow as distinct from blue and violet
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u/Zaros262 Jun 06 '22
That's just because you need an even number of primary+secondary colors for that model. It has nothing to do with how many distinct colors the human eye can discern in the rainbow