In most books I have read dealing with the Germans in North Africa, their uniform was made of an olive-green cotton material. This includes shirts as well, although one source stated that they were khaki. One other book states the tropical uniform varied in colour from brown to dark olive green khaki. Of course the original colour would bleach from the sun or as a result of washing, causing an off-white colour. My suggestion is base coat with olive green, and then depending on how 'veteran' you want your troops to appear, dry brush with lighter shades and then perhaps give a final wash with a desert shade. Oh yes, the helmets were painted tan.
from TIME LIFE books "Afrikakorps"