Icame across this info surfing the the web the other day..Racks question to me about the penned deer got my attention....this info belowseems to make sense..It looks like a couple factors play into over all antler color..they are stated below..

Oxidized blood/local plants - antler color depends partly on the amount of oxidized blood left over from velvet shedding and partly on a chemical reaction between the blood and sap from plants on which the antlers are rubbed.