I grow ' Carnival Watermelon' and find it to be a strong grower. The color fades as the leaves get older, but it keeps putting out new growth that is bright like it is in this picture. I think you're going to like it.

I have mine in mostly shade, so I can't say how the colors will do in the sun. I know a few others on this forum grow that one, so hopefully someone else will chime in.

I had my 'Autumn Leaves' in the sun at one point and found that its colors faded like yours was doing. I solved the problem by planting a peony directly in front of it. The peony takes the full sun and provides dappled shade for AL. You might try something like that if you find that your CW is getting too much sun. Perennials with large leaves work best. Just a thought. I have no idea if you have room enough to do something like that.

Here's another pic of CW that I snapped a week or so ago. You can see the faded older leaves at the bottom and the nice, bright new growth coming out of the middle. The newer the growth, the redder it tends to be. Lots of colors going on at once.