Some spectral colors are lighter than others.

[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

[Anon-C]

1. And here I thought rainbows had nothing to hide

[Jim__]

2. Are you asking why yellow is closer to white than red, green, or blue is?

I may be misunderstanding your question.

Here's an excerpt from an article on color:

Our eyes are detectors. Cones that are stimulated by light send signals to the brain. The brain is the actual interpreter of color. When all the cones are stimulated equally the brain perceives the color as white. ...

So, when all the cones are stimulated equally, we perceive white. At about 6:40 into this video:

He puts up a color chart of human vision, and it shows that when the green and red cone are equally stimulated, and blue is not stimulated, the color is at the greenish end of the color yellow and the yellowish end of the color green . So, that may explain the closeness of yellow (and also of light-green) to white. I know that different color charts will not exactly match this one, but he does talk about the fact that the red cone response actually peaks at a yellowish color - at about 7:30 into the video.

I would note that light-green also has little contrast to white. And when we are seeing a pure-blue, neither the green nor red cones are firing at all - so we might expect to see that color as far from white.

[Lionel Mandrake]

3. That's approximately my question,

but the video doesn't answer it.

[Lionel Mandrake]

4. The opponent process theory might be a clue.

It's probably not a coincidence that yellow is both
(1) the color perceived as lightest, and
(2) one of the extremes along the yellow-blue axis.

The video mentioned that Long (L), Medium (M), and Short (S) are the preferred names of the types of cones sometimes called red, green, and blue, respectively.

The video doesn't go into the physiological basis of the opponent process, but there are neurons that form the sum and difference of signals from L and M cones, and other neurons that add/subtract the S signal to/from the sum of L and M.

Different neurons compute

the "yellow" sum Y = L + M,

the difference x = L - M,

the difference y = Y - S,

and the sum z = Y + S.

Subsequent processing in the brain involves only x, y, and z, where x specifies position along the red-green axis, y specifies position along the yellow-blue axis, and z is the luminance (i.e., apparent brightness).

It seems reasonable that for a given value of z, the spectral color most easily confused with white would have the highest value of y subject to the constraint that x = 0. This is a hypothesis which could be tested by a psychophysical experiment.