What is it about the color yellow exactly that makes it subjectively less visible against a white background than say, blue or red?
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The color called “yellow” in English is specifically the *lighter* shades of a range of hues between reds and greens. A dark red is still “red” in English color categories, a dark green is still “green”, but a dark yellow is typically “brown”.
That said, that’s not a complete explanation, because even equally dark yellows look lighter.
The answer lies in the [opponent process](https://en.wikipedia.org/wiki/Opponent_process). You have three cells in your eyes – S, M, and L cones (for *short*, *medium*, and *long* wavelengths) that are sensitive in the blue-violet, greenish, and yellow-greens respectively, but your brain actually sees color by comparing differences between them.
* Brightness is the sum of all three signals, S+M+L
* “Yellowness” is (M+L)-S, with “blueness” the opposite S-(M+L)
* “Redness” is L-M, and “greenness” is M-L
Since both M and L contribute to brightness, and since yellow stimulates S cells very little, yellow is close to white in terms of the structure of your visual system (with two positive components and one negative).
Blue, on the other hand, looks unusually dark, because M and L contribute *negatively* to blueness but *positively* to brightness. And red and green have neutral brightness, because they have one positive and one negative out of M or L and ignore S.
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