Why does Uhura sometimes wear red and sometimes wear yellow?

Uhura only wore a gold uniform in "The Corbomite Maneuver" and "Mudd's Women", the first two episodes in which she appeared, very early in the first season of the Original Series.

I don't know if there's an in-universe explanation for the color change. Maybe she was in the command-track program and then dropped out? A likely real-world explanation, though, is that at that point in the show's development they didn't have the uniform color scheme (blue=science, gold=command, red=engineering/ship's services) all figured out yet.

I'm glad they figured out that communications officer Uhura should be assigned the engineering (red scheme)..Isnt communications/audio a division of engineering anyway? Science division blue is obvious with Spock and McCoy. I always considered the gold scheme as "navigation" personnel. The red Uhura uniform also adds a nice balance to the set anyway and looks great on her, imo.

Niether I believe she is a communications officer of some sort, I'm pretty sure. As for the color change I don't know, different uniforms.

she is a science and operations person so i think she has both