this, although some clan members themselves use salt and pepper skins (Anex being one of them since he posted snapshots of his mods on the forums before), skinning is by all means allowed, but selecting skins that would give you an advantage over others using said skin is not allowed.
I have seen my share of black and white, gi joe blue tinted and spiderman red tinted skins, which is fine IF AND ONLY IF the skin doesn't give u an edge over other players who dnt have said skin; for example, making transparent players more visible. note that in fact, if someone uses bright and colored skins, they will screw themselves over on bright maps when they have a hard time contrasting enemies to the bright backgrounds, like dust aftersunset.
Purely cosmetic skins or HD character skins (bcos lets face it, those source ones are pixelated nowadays) are allowed, but like i said, no advantage should be gained from any skin or weapon or texture etc...