This whole paint ball  idea is a little too intriguing, isn't it?!!?  
I mean its a defensive move that has real possibilites...

but here's one more:

How about one of those Stun Guns that shoots the electrode on a
tether, the degree of Crispiness to be determined by the offended. 
Now i don't think an occupant would feel the jolt (assuming we aimed
for the car, not the driver.  right?) but i bet 60,000 volts would
sure turn off his ECU for a spell.  When the car would die, it would
drift back from your bumper, pulling on the tether and releasing the
electrode dart, leaving nary a trace.  some field testing may have
to be done to verify this and to develop a effective dart for metal
(magnetic??) but i think it would be stitch to use.

-dan