”What's a good dry brush color for german field gray?”
Try Vallejo Field Grey and or the Valkejo Panzer Colours field grey(s) and highlight with green grey.
For my money the nicest German minis are the Crusader ones for early war and Artizan for late war. They mix well as the weapons are the same.
I recently bought the WG Germans in greatcoat, mostly to produce enough machine gun crew, sans rifles*. They are some of the best WG figures I’ve seen and they mix reasonably well with my Crusader figures.
* Pet peeve, nobody produces German machine gun teams with the right equipment.