By in large the most common paint scheme seen on German helmets during the war was rough textured field gray. For anyone who wants the so-called standard German helmet, textured field gray is what I recommend.
The German military did try to standardize helmet paint color during the war; however the color guidelines were never fully implemented. There were several reasons for this. Many of the companies who held contract to provide paint, were unable to fulfill their contract due to high demand and thus outsourced to other paint venders. To complicate matters further military supply depots often re-painted helmets with whatever gray-green paint was available. Anyone who has handled a large number of original helmets can a test to the shades of field gray varies greatly. I personally have seen original helmet ranging from slate gray to olive to almost khaki.
I currently offer three different shades of field gray. They vary from light to extremely dark. If you do want something more unusual let me know what it is and I will see what I can do.