1) Paint is impractical because it does nothing, and expensive because people do not paint stuff like that for free, particularly when mass producing the said item.



2) As I said above, the paint costs money while not being a useful feature.



3) There is no point designing and building a gun (or anything really) to be sold commercially when there is a better product on the market - For example, The AK, or any of the AK derivatives (The "copycat" models made by the chinese, estonians, germans and such).



4) All professional soldiers carry a side arm (like a pistol or in a few cases shotgun). Even if they ran out of ammo for those, they fall back from the frontline, find a Humvee or such and resupply from the said Humvee. Also, knives are easier to use when in you hand, not on a gun, because you have greater mobility and can stab/slash faster.