You should expect low temps in Montana, Wyoming, and Colorado to be in the 40s - sometimes high 30s in August. You could always mail the cool weather stuff via "General Delivery" to some place like Missoula that you are sure to visit and that you'll be there on a weekday (or Sat a.m.). Then you can mail the stuff home from Pueblo.
Depending on your skin melanin, long sleeves and long pants can also be very nice in intense sun. Also saves on the sunscreen bill and reduces the number of bugs that get stuck on your skin.
All good advice.
Just got back from riding the portion of the Trans Am between Missoula and the bottom of the east side of Big Hole Pass. Was also in Twin Bridges. Hit a warm patch and we still had two nights where it got down to 40 degrees. In ’00 I had frost on my tent in Yellowstone in early July. A flece could come in very handy.
Intense sun can also be a factor. I have an olive complexion and never use anything above 30 SPF back home in the east. For this trip, I used my GF’s 90 SPF and still got a lot of color.
Hopefully, the skeeters will be gone when you get where we were. Wisdom and Twin Bridges (as well as several other places we rode through in the state) were infested. You have to climb a little lip several miles west of Wisdom before reaching town. Forced us to hit “mosquito speed.” I looked at my GF’s butt and there had to be a dozen of them on her. And they do bite through spandex. Got to the grocery store in town. Before I could get my money out of my pannier they were on my legs and arms.