You have this interface, right? You can change paint mask by "/" and "*" in numpad, if you don't have numpad use numlock (look in google). You can change colour by numpad numbers and "shift" for gun/paint colour to change.
If you need to use your own masks you need tier 4 and GMS2 game engine. Change one of "mask" sprite.
Write your email, I can send you tier 4 generator if you have GMS2 engine