Bob ... I think it also depends on humidity. I can tell you hot humid sun is very, very different from hot arid sun.
I do try to compensate for that in how I plant and how I care for the plants.
Often, I'll site a plant where another plant will block the afternoon light, so the heuchera is not really in shade, but it doesn't get a direct hit of light either. Hard to describe.
I had read that the darker colors could handle more sun than the lighter colors. I need to put most of the darker ones where they only get just a little bit of dappled shade in the morning and very little afternoon shade in the afternoons. More than that, I've got crispy leaves. Who wants to grow something that you have to grow under a tent ? However, the foliage contrast does make for some really interesting combinations.
Another variable I have going is elevation. Light at higher elevations seems to be more intense ... depending on what side of the mountain your property is located on. We were doing a heating survey for my house and the computer program that was used was incredible in measuring the impact of light as a heat factor.
It's fun to learn something new like that and then seeing how it my impact garden plants, too.
There are heucheras that are native to the western US in more arid climates that can handle arid heat, but I haven't seen them in the lineage of those with the dramatic colored foliage. Of course, now the patents aren't disclosing the full linage, but stating "proprietary seedling" in the lineage trees, so it's harder to track it down.
