I seem to remember in the 60s that zinc chromate(sp?) was a primer for aluminum, and that seemed close.
Take one of the heads with you to the hardware store and see what is close. Most anything you find will be enamel which won't last if you shoot the head a lot. But for hunting it should only be shot once-right? Of course you could break away from the herd and paint them red, pink, yellow, brown, or? Pearson Switchblades were white, yellow, brown, green.