Utah, Arizona, Colorado weather

Depends on your comfort needs, and where within the states. I like the fall for all 3 states, especially post Labor Day. Less people, less traffic on back roads, more campsites available. Arizona in the Winter, is the home of Snowbirds. Also depends on where you want to visit. The national parks in these 3 states are packed from Memorial Day to the week after Labor Day. The Grand Canyon can be Snow-bound for the Winter months.
 
when it comes to Colorado, it's hard to say... most states are great to go camping in during the summer but we had snow in Colorado during June this year so its hard to really pinpoint a good time to go... best bet though? July-September, just remember to bring lots of rain gear, cold weather clothes & sleeping gear no matter what season you're camping in here in Colorado.IMG_20200609_111836_071.jpgJune 9th, 2020
 
Depends on where you go. 13k' in Colorado in June can have snow, where in Phoenix it can be 113F. Late fall in out of the ordinary places like Bisbee, AZ, Bryce Canyon, UT, are awesome.

IMO, best tiems to travel:

Ouray/Silverton/Telluride/Montrose, CO - mid June to usually early November (All mountain passes closed here a few days ago due to early snow)
Pueblo, Denver, Loveland, Estes Park, CO - mid May to early December on average
Moab, Arches, Bryce, UT - Year 'round minus parts of spring. Mud gets pretty nasty at times
Flagstaff, Winslow, All of northern AZ - late March through Thanksgiving
Phoenix, Tucson, Bisbee, Douglas, Tombstone, Yuma, AZ - End of September to end of April. Bisbee/Tombstone areas a little later due to elevation.

IMO - Fall is the best time to travel anywhere in the US. However, fall for everyone is not the same. Wisconsin has most of their leaves on the ground already. Here in Texas, we wont lose our until close to Thanksgiving. Then again, we just had two days of light snow and lows in the low 20's the past 3 nights in West Texas. El paso even got snow, which is rare and even more so in October.