Brad:
I have photographed Colorado in late September several times. I generally prefer the western slope and San Juans. Dallas Divide can be spectacular, as can the Elk Mountains southwest of Carbondale. The drive from Ouray to Carbondale, with a side trip to Dallas Divide is great. Don't forget the southern part of the state, east of Pagosa Springs and across Wolf Creek Pass. Even down into northern New Mexico near Chama. As much as I love Rocky Mountain N.P., it's not the best place for color. In any given year, you may need to move north or south, or to higher or lower elevations to catch peak color at any given time.
Glenn