Maybe if the bike left behind a trail of transparent colored bricks with the glow effect? But that is probably hard, too... and would look ugly.
No no no, bricks wouldn't work. If you had 5 people riding around, the brick count would skyrocket. I'm thinking projectiles (long, rectangular ones) with no velocity would do well. We can change projectile color, so that's fine. But could you program a vehicle to have a constant "onProjectileHit event"? If we could, this is pretty simple.