Fall is the time of year that everything around you starts to change color. This makes riding in Colorado even more visually stunning than it is normally and the desert in Arizona adopts a less hostile aura. The forests transform with resplendent leaves in the rugged mountains of Colorado and down to the rust-colored canyons of Sedona. Grocery stores even seem to acquire an orange hue in anticipation of Autumn; red tints erupt, ready for Christmas-time. It always seems absurd that Christmas creeps in so fast, but such is what happens with winter's march. This is a particularly interesting point if you do the majority of your cycling outside.
Cool fall temperatures (or reasonable temperatures for a Phoenician) leave many a cyclist cold from poor gear choice. Remember that if it has been over 100 degrees for literally forever that 70 degrees may feel chilly on those rides in AZ. In Colorado, the mountains have many chilly spots waiting to ambush the rider. Anywhere you ride by streams or lakes you could be assaulted by 15-20 degree colder air. Fall is the transition time and now is the time to break out the layers. Winter is easy to do in Colorado: you wear everything. Fall is trickier; base layers are always a good choice and many cyclists wear them year-round to wick sweat in the summer or provide warmth in the winter. In fall, most often the best clothing is still light but thin. Windproof is the key to chilly, not freezing weather. A solid windproof vest (even if only front is windproof), knee warmers or leggings with windproof knee sections, arm warmers, windproof gloves, and some warm socks with shoe covers can get the job done while allowing you to shed gear as the weather changes. Primal Wear makes a diverse amount of fall and winter clothing to help fill these gaps in your wardrobe. You can check out their various weather solutions http://www.primalwear.com/.
In order to maximize your fun this fall dress right and stay warm. You do not want to miss the last few weeks of amazing fall riding because you got Hypothermia. Crazy as it seems, Hypothermia, generally speaking, is when your CORE body temperature falls below 95 degrees, regardless of how cold it is outside. So err on the side of caution and use all your pockets to stuff extra clothes in.