I do
Basecoat black (real black, not "scale black" or "tire black" or "rubber")
Dump "Rust-All" on everything
Drybrush medium grey (I use "Soviet Dark Topside Gray" or something like
that)
Drybrush a LITTLE bit of gunmetal
This makes an okay, mildly rusty, somewhat grubby track, suitable for most
non-mud-covered applications. For a SERIOUSLY grubby track (Panzers in the
Ukraine or similar), I'd probably forget the metallic, and instead finish it
off by slopping on a mix of Real Dark Brown paint, fine mud
(locally-collected bentonite-- I think the Rust-All people sell it in a
bottle, but why pay for it when I've got a backyard full of the stuff?),
thinned white glue, and chopped-up Hudson & Allen "tall grass" fibers to
taste.
-m