I reckon I'll be in the market soon for an airbrush gun.
But for the time being I use a foam brush to apply the Walmart acrylic paint for stuff like invasion stripes on foamies. It works out purdy well but I dab the paint on the foam to avoid streaks in the paint. I'm almost certain the texture and finish would be far better than using a paint brush.
It would take take forever to paint the whole foamy this way though, which brings me back to the airbrush gun.