system
Given the following post, respond as if you're commenting on Reddit:
human
Why is it difficult to see yellow on a white background?

Assuming normal human vision, what is it about the human eye or the color yellow or both that makes yellow inherently more difficult to distinguish on a white background than other colors?
gpt
Perception of yellow and white relies on overlapping mechanisms in the retina.

Simplifying a bit: yellow is sensed by "opponent" neurons that are excited by medium (M) and long (L) wavelength cone receptors, *and* are inhibited by short (S) wavelength cones. So, when the stimulus has a lot of long/medium wavelength content but little short wavelength content, those neurons respond and you see "Yellow".

The "white" (or really, "brightness") signal comes from neurons that simply pool the L and M cone inputs (but not the S! - or maybe a little bit of S, sometimes). Call these L+M cells.

So, if you have a long/middle wavelength light mixture, it will look yellow *and* bright, since you're exciting both the opponent cells and the L+M cells. But then you can mix S wavelength light into it, and it won't get brighter but it will look white (since those opponent cells are now being inhibited).

Since both colors have such common neural architecture, they are perceptually very similar.

Meanwhile, blue and yellow look extremely different, because they excite almost completely complementary neurons, starting in the retina.