I've used liquid latex body paint that you can often find in sex shops and also order on line. Make sure the model doesn't have a latex allergy. Also, most of the liquid latex has a very strong ammonia smell. I've found that if you pour it out into an open container and let it sit for 5-10 minutes before you start painting, much of the offensive smell goes away.
I've also used kid's paint...finger paint...the stuff you can buy in any discount store. That stuff is safe for the little kids to eat, so it's pretty safe on a model's body. It's also washable, so it's pretty easy to get off. The liquid latex stuff just peels off...but it is hard to get out of anyplace where it's gotten into hair...and some models have told me it waxed them when it was removed.
===> I'll let her know to shave before the shoot