The only single cylinder Ballarat built Guldner engines I have seen were painted green, no doubt the same paint as Ronaldson Tippett's other engines of the time. However the imported multi cylinder examples were all painted grey as far as I know. Some engines may have left the factory painted a different colour at the a customers request or been re-painted to match a machine it was fitted to. I would be looking at the paint remaining on your engine.
Hugh