The thing about 40K, most of the combat is done by people with devices that make camouflage pointless, so you can pimp out your uniform all you like and it won't make you any more or less likely to be killed. Probably because most combat is urban anyway. Ignore those whiners who talk about "cameleoline" and stuff. They're just cowards waiting to be shot.
Seriously, though, I'd guess they have something like a cam-cloak like Gaunt wears or something when not fighting in urban environments.