Here's a thread which may help: http://forums2.gardenweb.com/forums/load/peren/msg0218300910368.html

The dark Purple, Maroon, Black hues are caused by a pigment, Anthocyanin and is produced by UV rays to perform several functions.
In a location with direct sunlight, it acts as a photoinhibitor to the intense bright light, so plants in that location would produce more Anthocyanin.
In locations with intense heat(yours and mine), less of the pigment is produced and the foliage begins to turn green(chlorophyll production).
Various tests have been performed to determine how to increase Anthocyanin production, especially in edible plants.
Some of the results indicate that Nitrogen(N) and Potassium(K) deficiencies can result in higher production of the pigment, so does the increase in the trace element, Magnesium(Mg). Also, low moisture introduced stress(drought), can cause increased production of Anthocyanin.

Loropetalum chinensis, var. rubrum and it's many cultivars performs best in total sun exposure, but some of the cultivars have better and more long lasting coloration.
The best one is said to be 'Zhuzhou Fuchsia', which "they" say will retain the dark foliage even during high temperatures.:Hope this helps.
Rb

Thanks to both of you! I followed the thread (not sure why it didn't come up in my initial search yesterday.)

If drought and stress help make the leaves dark, mine should have been black!! Mine have partial shade and I used gray water to keep things alive last year. My soil in that area of the yard was trucked in fill dirt, so is not as acidic as the normal clay here.

I'm guessing that the person in the thread that mentioned lawn fertilizer run-off as a potential cause may have been on the mark for my situation. My lorapetalums are at the low end of a slightly sloping 5,000 square feet of Emerald Zoyzia that my guy has looking like a golf course.

I'm thinking I'll try adding some magnesium now. Then in the fall, maybe planting some low growing perennials or annuals in front of the lorapetalums to try to catch the bulk of the run-off.

Maybe some judicious pruning will give me some new purple-y growth, too?

Does anyone see anything wrong with that plan, or do you have some other suggestions?

Thanks for the help!
Gayle

My Cotinus "Grace" started out a lovely plum color and is now almost entirely green. I was going to post a similar question, so I was pleased to read this. Our grass has rarely been fertilized, so I don't think this is the culprit for us. I will try adding some Mg and see if that helps.