What I normally do is ;
-Clean them real good with detergent and let dry completely. If you don't do that, the paint will lift off when you curl them on the wheels. Nasty looking
-Brush or airbrush entirely with Tamiya gun metal added a little flat black.
-After dry, soak it in Burnt sienia and Raw umber. Make sure the mixture is not to thick or thin. It's now your choice for the mix to be more on the reddish/brown or the lighter brown side. My own impressions, reddish/brown looks more for a vehicle that was parked in museums so it collects rust. Let it dry for 1 night.
-Now, slightly drybrush with metallic grey. I like using Tamiya enamel for this step on a flat brush.
-It's now time to fix your better looking rubber tracks on your model.
Drybrushing metallic grey on track surface simulates tracks contact to hard surfaces such as concrete roads or similar.
But if you wants to create a battle field muddy or dirt look. Paint or drybrush them tracks the same color as your mud or dirt as the hull. My English is not so good but if you close your eyes and imagine of your 4WD truck just went off a pond of wet mud. You can hardly see the blakening of your rubber tyre.
Have fun!! :>