Hello!
If the tracks are black vinyl, then after painting them brown like Rob said you could also lightly sand the outer rubber pads to reveal the black vinyl underneath - this gives a nice worn surface that looks a lot like real rubber.
It's a challenge to find paints that would really stick to the vinyl... On one track I had a good hit with MM metalizer, on other track it had a tendency to peel off :-( MM paints are no more, anyway... Some people say to go with Gunze C-series paints - sounds like it could work, didn't try it yet...
Hope it helps, have a nice day
Paweł