I also think that Denver proper and some of the suburban counties had outdoor mask rules, but I never went that far. Sure, I'd put one on to go into the grocery store, but that's it.
I also think that you don't have to wear one outdoors as long as you can maintain social distancing. I was never really in that situation where I couldn't get away from others, so I had that going for me, which is nice.
This past week Colorado gave up making regulations for the entire state and is letting the counties/cities make up their own rules. A lot of the rural areas have lifted all regulations.