Plisken I am certain as a fairly experienced inquisitor that faith is your shield. I have read, and I believe you will find a various quote perhaps in the IG codex or the Armored Divisions release pertaining to the fact that camoflauge is not a necessity with the Emporer's grace protecting you.
(Or is it in the rulebook? Hrmmm)
Anyways with that glory in life and honor in death stuff you guardsmen are always going on about I don't know why your worried about the practicallity of camo. Unless you've suddenly found it "glorious" to hide and shoot from covor. Whatever.
I'll let you in on a secret though. On some planets its just cheaper to paint lasguns with red, grenades with blue, and uniforms with white. Hope you don't get conscripted there. Good luck!