Most nations vehicles tend to be painted in a certain in range of colours within the same time frame. For the StuG IV it would be a dark yellow base with green and red brown on top. Your best bet is to do research into the vehicles of the nations you want to build and you soon learn which colours for wqhich nation in a certain time frame.
I know Vallejo Model Air has the 4 main German armour colours, i can't recall what they are off the toip of my head, but i know their website lists them.
I am a Norfolk man and i glory in being so
On the bench: Airfix 1/72nd Harrier GR.3/Fujimi 1/72nd Ju 87D-3