Panzer grey had a tendancy to lighten up quickly under the sun, so you may want to airbrush a distinctly lighter shade of this on it. Do not make it regular, sort of a cloud pattern if you will. Thin the paint more than usual and keep the application light. You'll want more on the top surfaces, less on the sides. As said before, don't make this uniform. The same effect can be created by drybrushing if you are uncomfortable doing this or do not use an airbrush.
A couple of filters of raw umber would probably be in order as well, followed by a pin wash of a dark brown, earthy color. A spot or two of rust? Sure, it's set in Russia. Chipping? Brick dust? The possibilities are endless.
True, Stalingrad did not have much mud in it but remember the vehicle would have had to travel through quite a bit of it to get there so some dried mud and dust would not look out of place. Anyhow, that's where my head would be traveling were I you.
Regards,
Mark