Id Rather the Enemy not be able to pick me out of a crowd as my captian. I mean if My captian is wearing a diffrent color from other people in my away team well.. One would think that the Klingons would say Hey look at the Guy in the green shirt.. Bet hes important kill him!... XD
hence why starfleet regulations prohibit a Captain leading an away team. Should be done by 1st officer... who is also wearing the color.
Actually this was a noticable problem in WW2 and prior. Officers were often marked on their helmet to siginify they were a person of rank. Today, the only way rank is shown is on a 1inch square patch on the chest(In the Army at least). Close enough for Gi Joe to see who's in charge, but not achmed the terrorist.