Vallejo makes specific colors...both of which I've matched up against other stuff (including some original Heer clothing) and they look real good...
Vallejo German Field Gray # 830 - matches Reanart's photos above
Vallejo German Gray #995 - is the typical Heer gray.
Hope that helps...but like everyone else has said, with the variations in dye, fabric, and of course, manufacturers of the clothing, there's some wiggle room.
Jeff