Are you looking for Aerosol, brush-on or airbrush?
Paints for foam planes depend a lot on the type of foam.
I'm not sure what the Radian uses - probably expanded polystyrene or something similar to Elapor.
Depron and most polystyrene foams will dissolve with most aerosol paints. Enamel paints (like Plasti-kote enamel that's easily found in Woodies, B&Q etc) in aerosols can normally be used providing you mist the paint on in VERY light coats. This is a bit risky though if you haven't tried it before and don't have some scrap foam to try it out on.
I've used aerosol spray cans a lot for painting Depron but not so much on other foams. I use either misted on coats of Plasti-cote or I use Styro Colours foam-safe spray paint. It's a bit pricey but works well.
http://robotbirds.com/catalog/index.php?cPath=292_130or you could try Richard at
http://www.expressfly.org.uk. He might stock some.
I don't think EPP has any problem with the Plasti-cote enamel but it's always best to try it on a scrap or somewhere inconspicuous first to be sure!
If you're brushing it on, I think you'll be safe with anything water based and most acrylics.
Same for airbrushes but be careful what you use to thin the paint. Cellulose thinners will almost certainly dissolve most foam.
Aidan