Sorry it took me a few days to respond, I'm working out of town and I've been off the net!
We had a group of shed hunters up to Alberta from the mid-west last spring, and this same topic came up over a few fine Canadian whisky's one night. The concensus after much debate (and, of course, a few more whisky's) is that this question has a 2-part answer.
In my view, the coloration of a buck's antlers has a great deal to do with what he chooses to rub on. The majority of the dark coloration in Alberta comes from spruce and pine, as well as what we generically call Red Willow. The pitch and bark are ground into the antlers, as well as collecting dirt and other matter. I think that as hunters we generally underestimate how much time a buck spends rubbing. If you disagree, take a piece of high-grit sandpaper to a shed and try to replicate the shine that a buck produces on the inside of his main beams, near the brow tines. I'd wager that you'd be working for hours before you even came close (believe me, I've tried it while restoring weathered sheds).
The other part of the equation, if you ask me, is where a buck spends his time, specifically during the daylight hours. All things being equal, the buck that beds on the open aspen ridge will have far lighter antlers than the spruce-swamp dwelling monster that rarely sees the light of day. The proof to this theory lies in looking at fresh sheds that lay in direct sunlight for even a short period of time. Sometimes they are so bleached after only a few weeks on the ground that you have to take a second look at them to see that they are freshly dropped!
I hope that this helped answer your question! As for me, I'll be looking for some beautiful fresh brown sheds between Christmas and New Years, we almost always find our first sheds during the holidays!
