The Austrian Berndorfer helmet was a slightly different shape to the German one. Luckily the Austrians acquired lots of German ones too (though whether or not Stormtroopers had them I do not know). The main rifle of the Austrians was the Mannlicher rather than the Mauser, but they did build Mausers under licence for export, and these were diverted to the army when the war broke out. So I guess you can justify that too.

Helmeted stormtroopers would be in the later "Field Green" uniforms rather than the bluey "Pike Grey" ones from the early part of the conflict. The colour of this uniform seems to have varied a bit from a straight grey to a greenish-grey. The Austrian stand-and-fall collar is a little higher and more pronounced than its German counterpart, but I don't think that would be very noticeable on a 28mm figure. The Austrian jacket has breast pockets (some of the German uniforms lacked breast pockets altogether as I recall) and the flaps on these have a distinctive style to them not seen at all on German uniforms (they are kind of.... pointy is the best word I can think of). If as I recall the figures are laden down with grenade bags then this won't be much of a problem as the place where the pockets should go will not be visible. Also the button arrangement is different.
Helmets were a light (slightly greenish) brown, I believe. There is a photo of some stormtroopers on the Italian front, and they all have cloth helmet covers on, which are a very pale colour, probably a sandy khaki shade.