The tracks are metal with rubber pads.
I painted mine flat black first next was a black and chrome-silver mix ( 1/2 - 1/2 ) then after that I made a redbrown and black wash ( redbrown 80% black20% ) and I made it very watered down also, this allows the chrome silver to show through a little bit and still keeps a dark black rusty look. after this I go back over and paint the track pads flat black and start making my pastel mixs up. My first pastel mix is for the rust, it's a few orangeish colors like orange/ a brown / and maybe a yellow (I'll scan color swatches of what I use in a few minutes ) then once I get them ground down or scraped off I mix them together and add some water to make a paste then dilute it with a lot of water or alcohol (alcohol is better, disapates faster ) then just soak it on what ever and it dries on the right places (so it seems? ) finnally make some dry brown and maybe some sandy color with the pastels and brush some on dry for a lil tank trail grime.
Here's a pic to refference from
( my trick for M109's, I'm a cannon crewmmber so I know these from the inside out or I don't get promoted, it also gets the senior personel off my butt)
one in the track park (clean)
I almost forgot there is also a black rubber type pad on the inside as well, it goes around the tooth on the shoe (the link).