wxup

I'm working on an old foam Spitfire kit and need help painting it. I have an airbrush but not much experience painting foam. What paint should I use? I need flat green and gray paints but using the little bottles of Tamiya paint on a 50" wingspan plane will cost a fortune. What can I use that's inexpensive, easy to find, light weight, thin enough for an airbrush, and sticks well to foam? Any advise you can give me would be greatly appreciated. I have no experience so please give detailed information. Thanks!