You could black anodize them. That will hold up under any conition.
I did not know that Doug's bolts were made of Aluminum.
Factory bolts were Cadmium plated to resist corrosion (rusting). Cad plating is a big "No-No" these days, but you CAN still buy bolts with Cad plate if you know where to get them and you don't mind paying through the nose.