Sorry to say, but a rattle can is not the best way to do this. I use an airbrush with the dirt/dust color I want to apply thinned and apply in such a way that it dries very quickly. Almost as soon as it hits the surface of the wheels and running gear. Colors I use tend to be light earth or sand colors. If it were Vietnam it would be sort of a Georgia red clay color. IDF maybe grey/sand. German in Europe, light earth and German in North African desert, Tamiya Desert Yellow. Really the color can be what ever you want, just needs to be applied with an airbrush to get the best result because you can play with the air pressure and the paint to get the right look.