In a previous post I wondered why, given that our color vision uses an RGB system, our languages consistently treat the non-primary yellow as more basic than blue and sometimes more basic than green. Well, sometimes you have to step back and question the givens.
The above diagram shows the sensitivity of the three cone types to different wavelengths of light. (It comes from Wikipedia. All I’ve added is the vertical lines dividing the spectrum into “colors.”) Once you see that the sensitivity of the so-called red cone actually peaks at yellow, the latter color’s status as an honorary primary makes more sense.
Now the question is, why is red considered so much more basic than violet? Being both a bookend of the visible spectrum and the region where the “blue” cone’s sensitivity peaks, violet ought to be as important as red and yellow put together — but, while red is universally treated as the very most basic spectral color, violet is not a basic color in any language. (Purple sometimes is, but purple is not the same as spectral violet.) In fact, violet is so unimportant in our color vision that it’s the one spectral color your computer monitor is physically incapable of displaying — and no one notices.