I was doing a five-day trip last August, and I was along the Great Western Divide at elevations to about 13,000 feet. It pretty much validated the clothing assortment that I had with me.
I started with ordinary nylon trousers, a synthetic t-shirt, a thin long sleeve shirt for sun, and a baseball cap with bandana for sun. Shoes and socks got wet during stream crossings, and they would dry out incompletely during the day, so dry sleeping socks are important.
As soon as I got into camp, I pulled on a 10-ounce synthetic fleece shirt. It rained on me each afternoon, so I then pulled on my 4.5-ounce hooded rain jacket. I don't need any wind shirt if I have a rain jacket. By the time I was cooking that evening, it was cooling off, so on went the down hat and down inner jacket. During the night, the down items were inside my sleeping bag. In the morning, the temperature had dipped to freezing point, so I everything was worn again. Halfway through breakfast, the down items were removed and re-packed. By the time I headed out on the trail, the fleece shirt came off as well.
If I expect the weather to be colder, then I add a down vest to the combination.