In Montrose and rides at that elevation (Black Canyon, Tabeguache-Unaweep, etc) temperatures at night in the 50s and during day 70s/80s. At the top of the passes, nights get cold, and temperatures average in the 50s/60s during the day, but you're only at the top of the passes briefly. Usually feels pretty darn nice!
Again, these are averages! Usually September is a pretty dry month also. Should see some leaves turning at the higher elevations!