Why would you cover over the active camo? It makes it leet skeet if there's an active camo receiver.
Who cares if it's painted or unpainted. Although most people do eventually paint them anyways.
Police will generally overlook that you have painted your receiver although the thing is if they're going to charge you that's just one more thing that you did illegal. Think about it, who's going to see it? If a police officer who's not playing on the field with you sees it, or they see and they don't request to search your car/trunk then you're doing something wrong.