No...seriously. Maybe charcoals and pastels? If you have a clear coat on the armor you could probably grind some up and rub it on with your fingers then wash away later. No oil based pastels, though!!!
Yeah I was gonna say dirt would work quite nicely if you want it dirty... If you do happen to use charcoals, be careful what you rub up against, as you'll be leaving black streaks all over your wifes brand new white angora sweater or the like...That stuff is ultra messy.