The few times I've been to Ouray, Silverton, and Durango mountain biking on portions of the Colorado trail have been the last week of Sept and the first 2 weeks of October. Aspens were in full bloom and colors, but one wind storm after a deep freeze will turn them all brown and blow them off the trees and onto the trails, where if they get wet, you may as well be riding your mountain bike on single track filled with marbles, very skittish and sketchy slippery to ride on.
Every year is different, you need a local ear to let you know when the colors are happening, and then just go, hard to plan it.