Nice work going on in here! Cam pointed me to you, said you needed some help with tires?
I use this stuff..Panzer Aces Dark Rubber
I know it just looks grey, but it isn't. I'm not sure what colors it has, maybe green? I dunno. It's a magic tire color

I just drybrush it over black.
For the dried dirt effect, I just mixed up some dry dirt pigment powders into a bit of water and slather it on, let it dry, and then seal it with Future Floor Wax or your favorite clear coat.
Hope this helps, keep up the great work!