Remember, velvet is very itchy for the bucks as it dries. The bucks rub the trees initially to sooth their antlers, and to polish up their horns. The color is according to the type of tree they rub. The color can be thought of as bone, the whiter the antlers, the more they have rubbed, and they have used harder trees. The softer the tree, the more residue from the velvet that is left on.