Take a look at Dicentra eximia bleeding hearts

Hakonechloa variegated grass
Variegated small-scale hostas
Perennial geraniums (make a nice mound of flowers and foliage)

Dwarf Aruncus forms a perfect little mound but wouldn't give you added color it that is what you want.

Hostas - Halcyon is a nice blue with not too big of a mound shape.

Jack Frost Brunnera.

Euphorbia or daylilies might work depending on the amount of sun.

There are also small shrubs that you could use:
Cool Splash Diervilla has a nice variegated foliage and does OK in shade so would work for partial shade. My Monet Weigela says sun/partial shade but mine (zone 5) seem to do better with sun. Great little shrub though.

I have big leaf ligularia in front of it and like how the leaves look against the Annabelle.

Ruth, Absolutely Beautiful

Thanks everyone for the suggestions, and Ruth & Firstyear for the beautiful photos. Keep 'em coming if you have more ideas! I'll probably plant in September so I have a while to plan still.

Ruth, two questions: how much light does your Persicaria get? My site is mostly dappled shade, but there's usually an hour or two of afternoon sun that might be a little harsh for something shade loving. Any thoughts?

And what is the other purplish hydrangea you have?

lalala- 'Painter's Palette' is something I have all over the yard, and it will be fine with that much sun. I have it in spots that get several hours of afternoon sun, and have some in almost full sun too. It can get tall, though, but I just trim it if it does. (The cut stems can be "planted" right in the ground. In shade and with some attention to watering, they'll root.)

The purplish hydrangea is Endless Summer. I had some leftover aluminum sulfate and applied it in early spring, but decided to let the soil be what it wants to be unamended. For some reason, the ones on the left are more purple than the ones on the right (which are much more pink than the picture shows).

I like hosta 'Great Expectations' in front of Annabelle/Incrediball too. This also has the persicaria in front (which I don't care for next to this hosta)...usually it's more covered in this spot, but after weeks of rain, the Incrediball blooms flopped a little to the left.