Maybe you'd like Vitex (link near bottom of list at left, after you go to the site linked to below). Does have shrubby tendencies, particularly in cold climates but also does grow into your desired size range in Texas. One in Brownwood was 22 ft. tall in 1982, another in San Antonio 19 ft. tall during 1991. (One in South Carolina was 30 ft. tall in 1984).

If you go their Vitex page, scroll down and click on second link The Next Mega-Superstar Plant for Texas, scroll down again there is a picture of one pruned into a tree.

Here is a link that might be useful: Texas SuperStar Plants

Magnolia!

If mess is a concern southern magnolia is definitely out.

"...at ground level, a wide tarn of black shade, and a summer litter of big leaves in ones and twos over a long period. To my eyes, this handsome tree has little place in home gardening"

--Schenk, The Complete Shade Gardener (1984, Houghton Mifflin)

Deciduous magnolias are another matter.

Here's an article on different redbuds...

Here is a link that might be useful: different redbuds

The other trees I can think of, in no particular order are - one of the citrus family, which would be evergreen; Stewartias; Cotinus, a.k.a. smoke tree, which has green, purple and pale green leaved cultivars, but you might have to do some limbing up to get a treelike look rather than a shrub, and it might be too small; Sourwood,; Carolina silverbell/Halesia carolina; one of the dogwoods; Franklinia; Amelanchier/Service berry; or Prunus mume, Japanese flowering apricot. Bauhinia, the Brazilian orchid tree, is also good.

A lot of the plums, including many of the purple leaved cultivars, can be prone to disease, so check carefully for the longevity of a particular cultivar in your area. And if Japanese beetles are a problem in your area, be aware that they LOVE the plums, and may strip the tree in the summer.

I don't think the purple orchid tree is a good choice. The flowers are pretty but that is where the good things about that tree comes to an end. It has some objectionable qualities.

If you're looking for easy, and durable there's crepe myrtle. The larger varietes could grow to that 20 tall range you were looking for. Comes in every color of the rainbow.

Since a crape myrtle 60 ft. tall has been reported (from Virginia) I guess one of the larger varieties certainly could grow 20 ft. tall. Heck, one in the unfavorable (for these) climate of Seattle was 32 ft. tall in 1990.

The flowering plum is an attractive tree. True, it isn't long-lived--I've had two, both of which lasted about 10 years .One replaced the other--that's how much I liked those purple leaves.

I don't remember any fruit problem. There are decorative plums and fruit-bearing plaums. Just make sure you get the decorative kind--may form a few fruits, but not many.

The flowers only last about a week, but like you said, it's the purple leaves that are its most attractive feature.

Kate

I vote for crape myrtle. I think 'Natchez' will look nice. Pure white flowers. I think 'Natchez' has the longest flowering period of all or one of the longest. Try it!

Crape myrtles, while very attractive over a long period in summer, do not, for the most part, cast enough shade to be classified as shade trees. Since the OP WANTS shade, I don't think CMs meet the needs.

Another small tree I had meant to mention and forgot to add, is a selected-for disease-resistance crabapple. You can also select for fruiting and how well it holds onto the fruits (if fruiting), so there isn't too much of a mess in the courtyard. Some of them have reddish leaves, at least for the spring.

'Forest Pansy' redbud would give the purple leaves, and probably be longer-lived. Try to get a grafted one, as seedlings are sold, and they don't necessarily consistently stay purple through the summer - some go almost totally green, and some just get greenish. Which means, in this case, that you should pay a higher price, as you do get what you paid for. Mine is probably a seedling, and it does go greenish as the summer goes on, but the price was right at the time....

I don't know if there are non-flowering, non-purple-leaved trees that you might consider, but birches come to mind - 'Heritage' or 'Duraheat' River birch might be nice, if a bit bigger than the initial parameters. I don't know if there are birches that stay small or not.