Taking as granted the fact that Nichelle really did look better in red than in gold-colored avocado, and moving on to try to posit an in-universe explanation, I would guess that communications involved electronics, maybe duotronics, certainly subspace technology, and so technical stuff. Uhura was certainly trained to repair and modify her own console under the hood, even though she hadn't needed to do it in years ("Who Mourns for Adonais?").
Why lawyers in red, too? Well, the law is very technical.

(No seriously, it should have been gold-colored avocado or arguably blue; historians should have been blue for sure.)
The kitchen sink is plumbing though, and we know Scotty knows all about that, so, yeah, the catch-all is red....
Interesting aside: security in Franz Joseph's TOS tech manual is in the command section (gold, with the star).