Yet red (in TNG, yellow) is restricted quite factually - it restricts what Eddington can do in his career. "You don't get to be a Captain wearing a gold uniform" is Eddington's specific lamentation on the fact that he cannot make Captain due to wearing gold, not on any esoteric desire to wear gold when making Captain.
Whatever the Trek names for the three levels, the colors would appear to mostly bear those out. It's just that Spock wears blue after already having had a stint in gold, and Uhura wears red after a similar stint - why accept "demotions" like that, even if Kirk after the "Where No Man" casualties is forced to fill certain lower slots with onboard personnel?
An alternate interpretation for the three colors is simply three shifts - it then becomes natural for the three top officers to all wear different colors. Its just that Kirk's Gold Shift sometimes summons specialist help from other Shifts (which all are of course active during the adventures which typically involve alert status and in general must represent a big departure from the passive boredom of the intervening weeks).
Alas, to try and relate the three colors to "departments" or "job descriptions" in a functional way just causes irreversible brain damage.
Timo Saloniemi