I don't know about Colorado, but out here in the Pacific NW, there are many, many areas--not just designated wilderness--with no cell phone reception. They will probably stay that way because there is insufficient population to support a cell tower. Last August, during the solar eclipse, my whole family camped together along the Metolius River near Sisters, OR. That area has lots of resorts, summer homes, year-around homes, and almost continuous campgrounds that are open year-around, so it's definitely not wilderness! There is no cell phone reception at all in that area. The few of us who needed daily access to phone messages had to drive out to the main highway to get reception.

I would never, ever count on a cell phone for emergency communication in a wilderness area! You may be able to get some reception by climbing a mountain, but you won't be able to do that in an emergency.

I always take a Personal Locator beacon for emergencies.