My experience is that they don't head for timber permanently until after archery.  They may get pushed into timber when raining or snowing (or perceived predation pressure), but if there is still food up there they will be back.  (Remember the dark, impenetrable "Timber" to us, is simply the patch of sub-alpine fir at the toe of the basin to a deer.  A rain tarp for the wildlife, if you will, just thirty seconds to a minute away).
One example:  last fall during muzzleloader (which, I believe is typically the week after the above timberline hunts) I had two different bigger bucks I had been hunting all week.  The Thursday before last weekend a front moved in with high winds at elevation and a couple inches of snow.    I was concerned the deer would take that as time to sink into the trees.  Friday, at 12,500 and daybreak I found them all again.  They hadn't left the higher basins.  Too many wildflowers to eat under that temporary snow cover with high atmospheric pressure building.  If that snow had come a couple weeks later and threatened to stay until June, the deer would be forced to head down.  They need to eat.
It has been my experience they go grey about the time they harden up.  Sometime in the last two weeks of September.  It seems that this is coincides with the first hard frost. 
As far as the early rifle seasons, I have hunted them once.  Got a decent deer, but there is so much attention to those hunts in Colorado.  I think they get shot out to a certain degree.   The unit I hunted had something like 20 tags.  If a guy has to wait a decade for a tag he will try to make the best of it.   From my perspective that is a lot of pressure for the bigger bucks. I think in allot of these units the expectations of a monster are a bit overblown.  
Since that rifle hunt I have been chasing muley's in under applied units, with a bow or muzzleloader.  I have had better success in finding older deer in units that don't allow rifle hunting until October. The deer fly under the radar in the units without so called "coveted" tags.  I covet any colorado deer tag I get, whether someone else wants it or not.
Hope this helps.