On the tanks etc. I've operated, the tracks are free of rust after a few miles, but I think a little raw umber or iron oxide color adds to the depth of a model's tracks. I've never seen a totally rusted track on a running tank, especially where the road surface, road wheels, idlers and sprockets rub them.
What I do is ask myself where the tank has been running, what would be embedded in the tracks such as mud and sand, and where it would be scoured clean.
Like Bill said, take a look at that other thread, it was very good.
Ron.