I would hope since they're outside, the lens cover would be durable. Otherwise a stone kicked up from a car would destroy it. But the paint being able to be washed away isn't really a problem if you're just trying to temporarily disrupt the camera's operations. If you destroyed it or permanently disabled it, they would probably very quickly replace it. But if you're about to do something you don't want the FBI to see, popping it with a paintball then later washing the lens cover would lend you a pretty reasonable amount of privacy.