Some spectral colors are lighter than others. [View all]

[Lionel Mandrake]

Spectral colors are the colors of a rainbow. These colors are pure, not washed out, not pastel, not muddy. Yellow is often said to be the lightest. Certainly yellow ink or paint has little contrast with white. Nobody would prefer to read text printed in yellow on white paper. Blue seems darkest, with red and green in between. Why?

One explanation I have heard is that it's all about luminosity, the peak of which is about 555 nm for people with normal color vision. But the color with wavelength 555 nm is not yellow; it's the color called "bright green", which is greener than chartreuse. Furthermore the fact that blue is much darker than red does not show up in a graph of luminosity vs. wavelength. So the question has not been answered in a satisfactory way.

https://en.wikipedia.org/wiki/Luminosity_function