Hi Don-

When painting foam, using a paint that won't dissolve the foam is your first priority! Some canned spray paints will make a melted mess of foam planes.

Acrylic craft paints are safe for foam, come in a huge variety of colors, and can be airbrushed. Krylon H2O spray paints are foam safe. Certain types of Testors and Tamiya spray paints are said to be safe, but I have not tried them.

Whichever paint you choose, be sure to test it on a piece of scrap foam before spraying the plane. Use very light coats to keep the weight down and slowly build up the color.

- Jeff

Edit: Two other Krylon spray paints, ShortCuts and Fusion, are also being used. The solvents in spray paints are what will melt foam, but people are having success by misting it on from a couple of feet away. Your mileage may vary, so test before committing.

Edit 2: Another technique is to first apply a coat of water-based polyurethane to the foam. This will partially seal the foam and make it less subject to being attacked by the paint.

As you can see, finishing foam planes is a big topic, and I'm sure you can find whole threads devoted to it if you do some searching. I am definitely NOT the expert.