See the Katex Documentation: Style, Color, Size, and Font for more information.
Note that: The last argument of the colorbox commands is text again, not math. You have to use $ again or else you get normal text font 
.
As colors you can use everything that works for HTML: names (see this table) or #RRGGBB hex codes.
Edit: Fixed typo pointed out by @helado
Niiice, thanks!
In your fcolor example, shouldn’t black come first and yellow second if its “first frame then fillcolor”?
Good catch! Thanks!
Very nice work, I got caught out by \color working as a switch unlike Anki (which uses MathJax). How did you find out that RemNote uses Katex?
It was discussed in Discord on 2020-09-13T00:00:00Z. So it was probably just somewhere in the back of my mind.
