In my opinion, I think the brush-on type works best. The airbrush pigments I see tend to be somewhat translucent, or thin...
I use WolfeFX bodypaint, order online... http://www.wolfefx.com/
has info about it. I also use Mehron "Paradise" bodypaint, Graftobia theatrical powdered metallic pigments for gold and silver, and Ben Nye sealer and fixative for the final coat to prevent smudging.
I use 98% Rubbing Alcohol to sterilize the palletes and brushes. I use paint brushes and makeup brushes for the bodypaint... and a 'trim' brush, like the foam kind from Home Depot, for metallic powders, which I sometimes mix with Mehron clear mixing liquid (stays on very nicely) or oils (baby oil, vegetable oil, olive oil... make sure the model is not allergic!) for a more shiny and liquid metal look.
It washes off easily with dish soap... might need rubbing alcohol if stains persist in finger cuticles, armpits, or other sensitive areas. Soaking in a tub, or working up a heavy sweat also help remove the pigments.
A full-body bodypaint project typically takes me 1 to 1.5 hours, with details, or maybe 15-20 minutes for full-body solid one color. One coat covers tattoos and is entirely opaque!
Absolutely question the model before painting! Ask about allergies or sensitivities to any of the ingredients in the makeup... especially if you are using other oils or ingredients, like peanut oil, coconut oil, etc.