The purpose of a wash is to get paint to settle into recesses, cracks, and crevasses and act as a pre-cursor to dry-brushing highlights... 99% of the time, you do a dark (shade of the base-color) wash, then dry-brush the high-lights with lightened shades...
Overall, I'd suggest using dark brown or dark grey for the wash.. Grey would probably be better, since you have a multi-colored camouglafe scheme in mind.. Even a "parade" camouflage scheme would be in need of the washes and dry-brshing, as the paint istelf is going to be grimy in places, but it will also be sun-faded in others, so keep that in mind.. Unless you're going for it rolling out the factory doors for the parade, that is...
Pin-washes go one step farther in the detailing, adding more depth to areas that are "open", as opposed to recessed, areas like grille-vents, intakes, exhaust pipe openings, machine-gun muzzles, etc., using thinned paint.... Time-consuming yes, but IMHO quite necessary.. An overall wash never gets those areas dark enough..