Why is Yellow Bright?

I was recently asked, upon informing someone that I’m about to start a Ph.D. program in biophysics, why humans perceive yellow as “brighter” than white - specifically, why it’s harder to read yellow text than equally bright magenta text (on a white background).

I said that’s an excellent question but I needed a couple minutes to think about it. Conversation moved on; but a couple minutes later, I announced that I had an answer. Not necessarily the answer, but something that at least sounds plausible. I’ve been unable to find verification, so let me know if you think I’m off-base.

We all know human color perception is powered by cone cells: photoreceptors in the retina that are sensitive to specific swaths of the visible light spectrum. The L cones are the most sensitive to red, the M cones the most sensitive to green, and the S cones the most sensitive to blue.

It just so happens that the blue-sensitive cones are by far the least common in the retina, and the difference between yellow and white is a matter of blue light (white is made up of red, green and blue; while yellow is made up of red and green). So it shouldn’t be surprising that we have a harder time telling the difference!