One thing I did try was having the stun-gun hurt you slightly every time you fired, and then the player regenerated health if you haven't shot for a second. The problem is that this requires a specialised player type (I used the Portal-Style Player) and wouldn't work too well in weapon form.
I still don't see the use of a paintball gun. The idea of paintball is to be able to shoot each other and compete without actually killing or seriously injuring people: In Blockland you cannot hurt other people in real life so using a "paint" weapon over a "real" weapon isn't any different.
I once managed to get the Bow to paint objects when you shot players/bricks with it, but as a quick test the paint wore off after a second and didn't actually hurt you as it just simulated the spray can projectile hitting you.