Some correction on the TOS Movies 2-6 uniforms.
Yes, the collars define the department one works in for the most part, but each collar does have distinct meaning:
[HIDE]White: Command
Gold: operations (helm; engineering)
Mint Green: Medical
Gray: Operations (Communications; Navigation); also Science
Dark Green: Operations (Security)
Red: Cadets, Trainees, Junior Officers
Light Blue: Special Services (e.g. Waiters)
Black: Enlisted & non-commissioned personnel
Dark Blue: Operations (ground forces? Memory Alpha isn't exactly clear on that)[/HIDE]