Fall is many Colorado hikers' favorite season, and for good reason. Temperatures are perfect - cool mornings, warm afternoons - without summer's brutal sun or winter's bitter cold. The daily afternoon thunderstorm cycle that plagues July and August fades away, leaving stable, reliable weather.
The crowds thin dramatically after Labor Day. Trailheads that required 5 AM arrivals in summer have parking available at reasonable hours. Rocky Mountain National Park drops its timed entry requirement in October. You can actually enjoy solitude on popular trails.
And of course, there's the color. Hiking through golden aspen groves, with leaves crunching underfoot and brilliant yellow canopy overhead, is a uniquely Colorado experience. The lower sun angle creates dramatic lighting all day long. September and early October are magic.