I went to Colorado in late October two consecutive years with the plan of backpacking with my niece who was in school there. Both times snowy and cold. The second time we said the hell with it and drove down to Bandellier in New Mexico to backpack:)
However, the first time we braved the weather, hiked through snow, and had a pretty good time, ignoring a little scare when my niece forgot to stake down the tent and it blew away into a thicket of willows with her pad and sleeping bag in it, us standing in snow, wind, and temps of 20 that night:) We hiked in the Lost Creek Wilderness. It's lower in elevation than much of Colorado, though we still got up to 11,000'. The lower elevations had a lot of cool sandstone rock formations, I recall. Mid-October could go either way, weather-wise for you, but you have to assume that it could be pretty cold and snowy. If you stay in the lower elevations of the wilderness, it will be a little better for temps.
I'm sorry, but I can't recall the specific trails we went on. I believe we started on the Goose Creek Trail, at around 8000', maybe were up around Tarryall Peak (?). It was a nice little 3 day loop. I got the original tip from someone on this forum. If you do a search of past threads from about 4 or 5 years ago, you'll turn up more info.
One other area of Colorado which sounds interesting to me for the shoulder season, but which I have never gone to, is the area around Grand Junction, Colorado Nat'l Monument and some neighboring BLM lands: lower elevation, more like Utah redrock canyon-type country. (If anyone here has been there, I'd be curious myself to know about it. I especially like that it's easily accessible by Amtrak) However, that's western Colorado, so pretty far from Denver, etc.