I cant speak to the legailty of painting or otherwise..
However, after you paint it, treat it as a real firearm.
Always treat it as though its loaded.
Dont point it at anyone, unless you are in a game/milsim.
Store it securely at home, keep out of reach of children.
Always transport it in a bag/box in your trunk.
Do not discharge it in public (including your backyard)
Do not display it in public (inlcluding your backyard)
In short, use your common sense.