I don’t know what zone you are in. I have Trachelium caeruleum (I think the variety is Hamer Pandora) which has dark red/purple leaves when in full sun, green with dark veins if in shade, and purple flowers. I’m in zone 10 but it is supposed to be hardy to zone 7. Mine gets a bit taller each year but is usually 2 feet or less.

How about Sedum 'Matrona'. It is a tad shorter than you want at about 16".

Sedum 'Purple Emperor' has very dark foliage but tends to flop when flowering.

I do have "Matrona" and love it. It tends to stay more on the blue-ish-green side of things for me, though. I saw some shorter sedum at the nursery -- something-something truffles (? Chocolate Truffles maybe) which had quite rich coloration. I'd like to avoid "Chocolate" Joe Pye because in this particular area the self-seeding is going to be a PITA; I have some on the other side of the house where I can easily reach into the bed to pull seedlings and keep things tidy, so it's fine there.

Moist might be a bit tricky, but truthfully I'm really not sure. My soil texture is beautiful and well-draining; it's not dry, though, but I'm not sure if it's actually on the moist side. I guess I could plant the stuff and find out. Which I'm probably going to do with a Filipendula "Red Umbrellas" that came home with me yesterday --> I could not resist that foliage.

Love "Vera Jameson" sedum, too -- but too short for what I'm looking for.

You will really like F. 'Red Umbrellas'. It comes up very early and never disappoints. Great foliage and flowers, what's not to love.

I planted Gladiolas last year and ended up loving them.

The color choices are plentiful, they're easy and reasonably priced. They came back in my borders this year . They're architectural, too.

I can say the same for Dahlias--love them dearly! They dark foliaged ones are awesome also..

mxk3, I went out to the yard this morning to show you all my red to purple foliage, mid-height plants and realized I really don’t have many. As discussed, there are lots of sedums that might fit the bill like ‘Karfunkelstein’ and ‘Purple Emporer,’ below is my ‘Chocolate Drop’ but the leaves right now are more green than purple. It will darken up some but it’s not quite as tall as you want. Another plant for your full sun spots that gets to around 3’ and I’ve touted repeatedly on here is Heliopsis helianthoidea var. scabra ‘Burning Hearts’, an amazing false sunflower with dark red to purple leaves and yellow-orange blooms. But again, it’s not the color you want and it blooms late June to September so may interfere with your existing scheme. Anyway, here are the ‘Chocolate Drop’ and ‘Burning Hearts’ both now and last summer:

The only other mid-height plants I can think of are the previously mentioned dark-leaved Actaea, which would work great in your shady spots, and the Chocolate Joe Pye you don’t want due to reseeding (we just got one last year, hope it doesn’t reseed too much). We have lots of Heuchera and Heucherella that would give you that foliage color but alas, they’re too short. Quite a puzzler.

Yes, a puzzler indeed. I could use Persian Shield - now *that's* an eye-catcher of a plant - but it's treated as an annual up here so I'd have to replant every year, and I'm not aware of any seed sources.

There are some red-purple foliaged Geraniums. I think I have Knight Reiter or something similar namewise.

I like my dark foliaged Actea (used to be Cimicifuga) as Peren,all mentioned.

Perhaps some of the taller dark foliaged Heuchera would be large enough?

Not formally perennials, but some of the more recent, smaller Weigela might fit the needs such as Fine Wine.