A water-activated powder that when applied is invisible, but when sprayed with water looks like the result of a motor cycle accident. Drag a wet knife across powdered skin for a bloody cutting effect.
Dust it on with a powder brush and then drag a wet butter knife across it. It looks like you're making a scalpel incision. Dust it on and then use a water spritzer bottle to spray it and it'll look like road rash. The possibilities are endless.