Hey guys,
This may sound really old school (but then again, so am I
!) but I've found nothing better than the oils that I use for painting the rest of the figure. You can mix an unlimited variety of shades, and blend them while on the figure into a beautifully subtle staining, dusting or whatever.
I often wait until the basic uniform colours are (absolutely) dry, which keeps the "weathering" from picking up blues, greens or reds from the wet uniform colours.This allows me to use a thinner-moistened brush to remove anything that looks wrong without damaging the underlying colours.
Or conversely, I'll take my "dust" colour (usually some combination of raw umber, white, burnt sienna, black and other earthy tones) and blend them into the still-wet highlight areas on the uniform, boots, etc. which tones the highlights down for another stained effect.
Hope this is of some assistance.
Cheers,
Brian