I don't have ATW myself (a crying shame, I know) and an online search for helmet images did not find any that fit your description, so the only thing I can go off of is my knowledge and experience of RTW.

I would venture an educated guess that the helmets are supposed to be bronze, but the lighting was off, thus creating the effect of a darker portion.

I could just be full of crap, though. Like I said, I do not have an example to study.

Thank you for your reply I thought it may be paint on bronze but brown is not a colour used for decoration.
Persion Invasion by comradgeneral has hoplites from different states with different shades of brown.
connorsgrandad