This i going to sound stupid, but trust me, it works. Find the cheapest copper colored spray paint you can get. Wally world type. Not VHT. Like the dollar a can stuff. They will look horrible at first, but after the copper burns off, they will be left looking a dull black. The collectors will take awhile to change color, but end up looking good too, IMHO. Just a thought....
(If I was messing with you, or joking around; I would have said to use hot pink or something like that. Seriously, the copper works....)