RE: Painting Foam??
Any water-based or alcohol-based acrylic paint is safe to use either brushed or airbrushed. Alcohol based acrylics can thinned with 91% isopropyl alcohol (denatured alcohol) that you can buy at any pharmacy.
Avoid spray paints, laquers, and enamels. There are some safe acrylic spray paints, but if it doesn't specifically say that its safe for foam on the can, then it probably isn't. If you're unsure, test it on a styrofoam cup or a scrap piece of foam before you try it on the plane.