|No computer can hit that!
Starting on a matte surface, dampen the panel with white spirits (it doesn't need to be soaked, just a barely visible sheen is enough). On your palette, mix your preferred oil paint with some thinner into a thin slurry (I used 502 Abteilung Faded UN White, Engine Grease, Faded Grey, and Faded Navy Blue). Get a little of the slurry on a clean brush (soft small flats work best) and apply it to the panel, working it over the surface making sure to leave no brush marks. Go around and pick out different panels with different colors for a nice mottled effect!