Some suggestions concerning color. In my experience what helps to bring a model to life are contrasts in color. Those will be dictated by a number of factors...age of vehicle, theater of oper, unit, etc. At first glance your STUG appears to be new with a fresh overall paint scheme & new tracks. But then your drybrush makes me think that you are looking for a vehicle that's been out and about for a bit. Please let me know what your intentions were.
The first step I would take would be to add a dark wash. This really sets off all the angles and shapes of the vehicle and helps disrupt the monochromatic appearance. Secondly, age(rust) your tracks. That would be a very interesting, easy, & accurate contrast. It doesn't take much time for bare metal in the weather to gather at least a light coating of rust. My next step would be to weather it a bit. What you have to remember with these vehicles is that they are machines with men constantly moving over them, grease/oil leaks, spills, & stains. Paint chips off. Dust and rain leave streaks. There's any number of approaches you can take with this. I find constuction equipment(loaders, bulldozers, cranes, etc) to be a great reference for this type of wear. Aside from being shot at they are worked basically in the same manner.
Once again I got carried away. PM me if you're interested in addition ramblings. I do like your piece and good luck with future projects!