real flight suits are olive drab because SERE and stuff I guess

but if you get blown out into space and survive, stealth is the opposite of what you want. you want to stand out as much as possible so that somebody, anybody can get to you before your oxygen runs out.

but then again while typing this i remembered that the x wing helmets weren't sealed like the TIE helmets were, and their suits were all black, so I guess the real reason is because George was doing color theory and wanted to make the two teams stand out visually from one another.