Airbrush is out of my field of experience, so hopefully some of the airbrush painters will chime in, however, I use the rattlecans, and when trying to come up with a rust color, I'll use Tamiya PS14 copper paint, it's got that light brown look, with some metalflake in it, to leave a rusty look when the outside colors get scraped off.
I happened to check out Spazstix and Faskolor paints, and unless you know the colorchart, and know how to blend paints, most of their colors are bright and shiny for some pizzazz, on the other hand, Tamiya has acrylic paints for airbrush. Check out their on-line catalog, there's an XF28 Dark Copper, and an XF6 copper, that if you experimented a little, you should be able to come up with a pretty decent rust.