I see that the rubber rims of the inside roadwheels are not painted. If you use an airbrush I'll tell you what I do to simplify it.
I assemble all the roadwheels and the tracks before painting, and paint all of it with flat black, being extra carefull to cover everything, leave no plastic visible.
After that I paint the inside part of the roadwheels, taking care to leave the rubber black. Then I paint the tracks with the help of an index card, to avoid overspraying the road wheels.
Finally, retouch the rubber parts by hand brush, and you are done!!and no one will notice the shortcut!