and not tracks that are all metal like the Stalin or King Tiger (versus tracks with rubber like the Sherman)
Most people (including me) seem to use a chemical blackening agent. Blacken-it is one brand. I bought a bottle of a similar product also from AK Interactive. AK also has a colored wash to apply over tracks.
With these methods you need to clean the metal with something like vinegar or lacquer thinner. It is hard, sometimes, to get the solution to flow into deep crevices and recesses in the track pattern. I use a fine paint brush or tooth to hit those spots. You can cover them up too with a bit of paint.
The tarnishing solutions give a nice dark, chakly, brown finish. You can dry brush them, add pigments or washes, etc. as normal.
AK has a tutorial video on their web site too. Although it uses their own fluid, the process is the same for Blacken it.