Paula, Susan - You will be in the Rocky Mountains, where any type of weather can happen at any time! In August it is mostly warm during the middle of the day, although sometimes a thunderstorm can come through and cool things off. It can be cold and/or windy at night and in the very early mornings. Take comfortable closed toed walking shoes, a pair of walking sandals, socks, some long pants, some shorter pants, a few long and short sleeved tee shirts, and regular shirts, mostly short sleeved but one or two long sleeved.. Have a warm fleece and a windbreaker which can be worn together or apart, and throw in a pair of cheap knit gloves. ( I have never needed my long underwear there in August, however.....I always wear it at least once or twice if we go in early June or late September I usually take it anyway, whatever time of the year we are there, just to "be prepared." Take a bathing suit, too.
Remember sunscreen, for at those altitudes, it is easier to sunburn. And make sure you have insect repellent, too. I often buy a new sweatshirt from one of the national parks while we are there, and enjoy it for years afterward.
America IS beautiful! You will have a wonderful trip.