Why are higher level math textbooks almost completely black and white? I can't think of any math textbooks on a subject more advanced than calculus that uses colors. Edited to add that the comments by Dave L Renfro and J W include examples of books that do use color. I have to imagine the use of colors has some educational purpose or psychological function, but I can't seem to find studies about it applying to age groups past K-12 (I'd be happy to read if you can point any out to me).
An exception to the presentation of advanced math content with color may be in video format, so there must be some visual appeal to the use of color. I just can't come up with the reason why this is not applied in books, even when printing is not an issue (as with ebooks, for example).
Historically, I imagine it was simply cheaper to print in all black and white and maybe people just prefer what they're used to. But it doesn't explain why even books offered in electronic format would stick to this rule.
From a practical point of view I understand that mathematical concepts don't have associated colors and don't require them (outside, maybe, of coloring results in graph theory and knot theory, where often enough colors are just represented by symbols).
I've heard the argument that colors make work look "unprofessional" or "inelegant" but I'm also questioning why that is, since it can be a largely subjective assessment.
I'm biased because I like using colors. When I take notes or write on the board, I like color coding, highlighting, and in general having quick visual "shortcuts" to find information on the page. I understand this can be distracting for others and I'd like to better understand why.
Edits: adjusted the question to say "most" as it is clear there are some counterexamples.