First, I wouldn't think I would put too much mud on it for the Iraqi desert.
I'd give it a wash with a burnt umber oil wash.
Then I'd drybrush in three different steps.
1. Hard drybrush with 75% body color and 25% white
2. Lighter drybrush with 50% body color and 50% white
3. Very light drybrush (just the raised sections and corners) with 25% body color and 75% white.
Give the lower hull a "dusting" with your airbrush and a "dust" color. The object here is to lay down a "mist" of color, but not totally obscure the base colors beneath. Work this up the sides of the vehicles getting lighter as you go farther up the vehicle.
I'd use wheathering powders (a dust color, or a brown) around all fine details to pop them out.
I hope this helps. Good Luck on your build.