I'm currently doing a Humvee in Nato camo scheme. The vehicle depicted will be 1st Marine Division in Iraq in 2003. Would it be appropriate to use a lighter color pin wash to show sand/dust build-up? Or should I do a dark wash and then mist with a lighter color?

Thanks

I'd go with the lighter wash. In fact Steve Zaloga used this method for his Dragon halftrack kit in MM, a sandcolored wash and dark drybrush (darker than the painted surface) to show scuffed paint etc.
See:
http://www.missing-lynx.com/gallery/usa/m2halftracksz_1.html

Thanks Marc.