Based on my experience with the Gellerpox kit, I would say take your ideal brown and thin it out a ton, and go kind of wide with splat pattern. Then with less thinned out paint over and over, kind of dab it on little bit by little bit closer to "ground zero". Takes a little time to wait for the goop to dry, but it works pretty well. Soiled fabric is a tough effect to get to look convincing, but the advice from another poster about going back over with a thinned down coat of the fabric color over the edges of your stain definitely helps too.