[Mr Modifier]

When I was a kid we used to black wash aircraft, tanks and figures with the residue in the tiny jar of white spirit that we cleaned our brushes in.  The dark muddy brown super-thinned “paint” would settle in panel lines and gather around rivet details and belt buckles etc. and when the spirit evaporated what was left was dirt stuck in the nooks and crannies of the model.  The idea is for the darker colour to settle in the low places and leave the raised parts completely clean. So if you do make a wash - make it very thin for best effect.

It only works if the paint you cleaned off was dark and some of it was matte.