I tried a UA long sleeve summer baselayer on a pass climb on a hot day. Halfway up the climb, I realized I was getting badly dehydrated even though I was drinking steadily from a Camelbak. I peeled the thing off: it was so wet I almost could wring it out. I drank a good bit while stopped and continued in my unzipped SS jersey with no further problem. Not recommended.

OTOH, I wear sun sleeves on sunny days, mostly to prevent skin damage which they do very well. They are a little warmer than without but the area they cover is small enough not to be a major contributor to overheating or dehydration. I also wear a light poly beanie to prevent sun damage because I'm bald. Again, the area is so small that it doesn't seem to be a negative thing. I don't wear a sun garment on my legs: too much area and sun screen seems to do the job fine there.

I've done long pass climbs in 100+ temps with unzipped SS jersey and sun sleeves just fine.

And, except for shorts, don't wear anything black.