I've been wondering about this and, rather than re-watching the entire series to find out, I figured that certain posters here might have already given this some thought.
Seems like the colors of the doors off the corridor set on TOS were a bunch of fun colors. Do you suppose that the colors were meant to identify what lay beyond the door in a similar spirit as having the various department crew wear different colored shirts? If so, what are the correlations?
I seem to recall doors that were red, yellow, gray, and blue.