If we're talking Colorado, that excludes grizzlies yes?
As for mountain lions, I live in the black hills which has a fairly dense population of them, but National Park Rangers, National Forest Rangers, and long time residents will all say that lions will go out of their way to avoid humans. It may be different if little kids are out and about, but I hike out in the middle of nowhere with no fear of lions, even at night. Not sure if the Colorado variety are any more dangerous.
It is also my understanding that lions aren't especially tough. They don't really want to tangle with something that fights back, so a gun of almost any caliber is a good deterrent. A bigger issue would be having time to draw, aim, and hit one before it was on you (if one did attack).
I wouldn't think black bears would pose much of a threat either, but they would be tougher. Still, I'd feel okay with a .357 loaded with 180 grain hard cast bullets. That would allow some lighter weight, trimmer firearms options that would be more comfortable to carry.