You can participate in any event with whatever weapon you want as long as it doesn't shoot too hot (fps too high). I would paint it before going though, the clear plastic would be a bit of a giveaway on the field.
But if anyone told you you weren't allowed to play due to your gun, they would be wrong.
Short answer. Yes. You can play if it is painted, or not painted. But everyone who plays will either have an all black gun, or one that was painted. No clear on the field.
I love freedom and consequently America