I don’t know if it makes physiological sense or not, but:
In acoustics, an octave is a doubling of the frequency. We can hear notes covering a range of several octaves - around 11 octaves. The neato thing about octaves is that two notes an octave apart sound remarkably similar, even though the pitch is different. So any “C” on a piano sounds like a “C”, and playing two of them together sounds like one note kinda slipping into the other.
We can’t quite see an octave - almost, but not quite. The lowest “red” we can see is around 400 THz, and the highest purple we can see is about 790 THz. I wonder if we could see past an octave, say up to 1200 THz, would “red x 2” look like a remarkably similar red?
No, it wouldn’t. What makes an octave work is that it matches the harmonics of something vibrating in one dimension, like a column of air or string. Vibrations in higher dimensions have harmonics too, but they don’t have simple frequency ratios like that – consider for instance a drum, which does not give a defined pitch.
The colors we pick up are based on changes between energy levels in molecules, which are basically 3-D vibrations, and really there is a single transition that we detect. Different proteins shift that to low, medium, and high energy versions. We sometimes call those RGB but really they correspond more to red, green-teal, and violet. We never see them at full saturation though because real light always sets off all three receptors some.
Then you can take three lights and mix them to copy some portion of what’s possible. For instance using RGB lights, we can use R+G to make yellow or orange, although not as saturated as the single wavelength would be. Then R+B makes purples, including a desaturated violet if you balance it right – when the amount of low energy absorption from red just matches the amount of middle energy from blue, leaving only an excess of high energy (the red is there to help make white, basically).
While I’m talking, I’d like to say I think indigo is unfairly maligned. The problem is “blue” covers an especially large range of hues, from jeans to the sky. Now we usually think of the color of jeans first, and call the sky things like “teal” or “cyan” or so on (but still “sky blue”). For Newton blue was the color of the sky first and so he called the other “indigo” after the dye. The shift has left it confusing…but they’re still easily as distinct as say orange and yellow.
