Honestly - and I hate to say this, because I always find it dissapointing - I think this would be a reskin.

I mean, what do you do with a paintbrush? You can't have it change the colour of enemy players - that would be too overpowered, even if it was backstab-only - and having it be a joke weapon that does little-to-no damage would make it useless.

+On kill creates a Decoy of you who runs around randomly
-15% health on wear