When I was looking for wrecks inspiration, I noticed that the powder from the rubber rims on the ground is common occurence. It looks impresive and give more realstic view of burned out tank.
I used two pigments: black and white. The first layer was black color applied on the ground around the the wheels. Then I tapped white powder, which gave the main structure. I supposed that it will be enough, but not in this case. The colors were mixed in two containers, so two additional tones were achivied. They were applied in several places. The result you can see on the photos below.