You cant actually get the node that the projectile hit, as it hits the collision box, not the model. The paint can projectiles just estimate based on z offset, eg if its less than 0.15, then change the feet colors, etc. You'll have to find these values yourself, or you may be able to call the paint projectiles function which handles this (which is probably the better option).
Run a trace, spray someone, and see what function handles it.