The overall base paint I use is Model Master Acryl 4812 US Army/Marines Gulf AR 1/2. I have had good results with Windsor & Newton Burnt Umber and Black oil colors diluted with mineral spirits. This next trick may sound dumb, but for drybrushing with a desert scheme, I have been using old Mary Kay makeup donated by my wife. If you accidentally put it on a little too heavy, just wipe it off . You can rub it into the base paint coat and get nice smudges over a large area without damaging or permanently altering the base color beyond repair. The color I like to drybrush with is a bronze shade of brown. It works really well for streaks of dust around the bolts, etc. The burnt umber wash is oil based and a little too dark for drybrushing. If you can get your hands on the July 2003 FSM issue John Plzak put together a really helpful demo on weathering and painting.