Melissa I just answered whaas similar but about burgundy. But some of these are the really dark. I'm going to include a photo here. I take awful photos and this was a day that was so overcast but I think you'll get the idea. It's ALL day full sun here. Prince on left, Dark Secret on right, 2 Caramel, and bottom is a Silver Scrolls.
Cher

{{gwi:731008}}

My friend wanted to do a two colors purple and peach color scheme at her front door. She put two terracotta pots on either side of her door in one pot Caramel and the other pot Southern Comfort in the center was Purple Spikes. Very pretty all summer.

This arrangement was on a front uncovered porch, full blazing sun, hot concrete area. They were left out all winter. Of course the purple spikes did not survive they are annuals but Southern Comfort was beautiful all winter and it is coming back strong this Spring. Caramel survived but is small. Hanging on for dear life. It does look like it might survive though.

We had a very mild winter and very little snow. Also, we had the hottest dry summer ever.

I have no personal experience growing heuchera in full sun, but I just got done perusing all the heuchera descriptions (per marquest's suggestion) at ilovehostas.net. Here are the dark ones that they say can be grown in full sun:
Blackout
Obsidian
Plum Royale

Christine

I water mine well all summer into fall which helps a lot. My Purple Prince is in half sun as is Citonelle but I have very little area that is full sun. My dark ones seem to almost hold up better in the sun and heat than the paler ones.

There is a gas station in the next town that had 2 palace purples growing in a 4'x 8' raised planter in their parking lot. It got sun all day until dusk. It got the heat from the pavement and the road. Salt spray from winter probably hit the thing too. They were the biggest Huecheras I had ever seen. They went for several years with no real care and then dwindled.

I'm surprised that Plum Royale was recommended for full sun. It burns in the sun for me. I moved it to a very shady area last spring and it is happy. The shiny purple leaves are beautiful.

Palace Purple is the only one that would be tried and true in full sun for me.

Cher, digital pics sometimes need to be adjusted to remove haze. I think it is noise - and some cameras remove it better. Below is your pic with a one click adjustment (I'm Feeling Lucky) using Picasa3.

Paul

This is where I think this whole zone thing is wrong. My sun in PA and the sun in Virginia is different. We can both be a zone 6 but the sun is stronger in Virginia plus we have more cloudy, partly cloudy, partly sunny days a year.

It will probably have to be trial and error to see what will work in any garden.

Hey nice Paul! My camera doesn't have a lot of settings, I just went through it. A photographer I am not. LOL I took that one at a bad time because I wanted advice about pruning the Spruce that day. I will work on better photos this year. Thanks a lot.
Cher

I've had Obsidian in full sun for about 5 years now. It is amazing and seems to be one of those Heucheras that never needs any care. It looks the same in the spring as it did in the fall. It has always stayed a nice mound and never gets straggly. Highly recommended.

I have Obsidian and Blackout in full sun. The Obsidian (3+) look great all summer long. Blackout is fine, but I think it looks enough like Palace Purple (bronzy rather than purple) that I will not go get more. I have a Midnight Rose in same location: it also look very good. Also Stormy Seas, though that does not seem to do as well there.

I've decided that Obsidian is one great heuchera. I'm going to give up on Plum Pudding for good, and put Obsidians in where all the Plum Puddings died (part sun, not full sun).

I'm in the Twin Cities and just moved a Stormy Seas from a mostly shady area to bright sun. It has perked up considerably in the sun already and actually looks like it will now thrive.