Sherman Track colour?

I start with PolyScale oily black. It has a bit of a metallic look to it. Where you go with weathering depends on the look you want. If it is a desert scene, light color chalks would be in order with some silvering showing wear in the right spots. A wet environment might show some mud (a wash would suit this look better) and rust would be appropriate. This is just a quickie, I know someone else will chirp in with more detail.