using shaving foam later. But is there an exact measurement of food coloring per can that can be used without the skin being stained?

The skin is an amazing organ, and it takes a LOT to stain it. As long as you're not using acid, or anything that is extremely hot (creosote, tarmac, larva, hot oil, tattoo ink), the shit you're doing won't stain.

Another answer is, I did a pie session using shaving foam and food colouring, and it came off her skin and hair very easily.

Use as much as you need dude

Thanks for the advice.