For Colorado in general, I would suggest a 20-30 degree bag - a classic 3 season bag. "Fall" weather in Colorado can start around Labor Day, honestly. It can start to get quite cold at the elevation of the CO trail by then (Southern terminus, especially). I really enjoy the Sierra Designs Cloud 20, and 35 bags. I own the 35, and have used the 20 when guiding in then High Sierra (CA).
https://sierradesigns.com/cloud-800-20-degree/I've detailed the sleep system I used (and continue to use) on a 2 month bikepacking trip throughout Colorado, sleeping at elevations from around 6,000' to 12,000' between July and Sept. Those September nights got pretty borderline cold, for sure, and it would have been nice to have something other than a 35 degree bag.
https://justinsimoni.com/2017/06/19/my-sleep-system-for-the-tour-of-the-highest-hundred/There's lots of strategies to "extend" the temp rating of a bag. I use a very high R rated sleeping pad. I'm probably going to invest in some puffy pants for next season. Using a sleeping bag liner is also another option.
