As minigun said, colour blindness is a bar to entry. Although there was a chap who got thru and even made despatch crew commander with colour blindness. Of course his nick name was 'top light-bottom light' as that is how he knew the difference between red on and green on. He was eventually busted and thrown from the unit.
Standard MES is required (ie fit) as the guys are classed as operating crew as opposed to aircrew.