I like lamium maculatum. It spreads but new plants have different colored leaves. Flowers are pink and are blooming now. I started with one plant and now my garden fence is bordered with its many colors of leaves.

I am in a lower zone, so you should check these out locally, but they have all looked great for me all summer:
Dianthus 'Greystone' (trim off bloom stalks after flowering is finished) - sun
Cranberry/Vaccinium macrocarpon - sun to 3/4 shade
Veronica 'Georgia Blue' - sun to 3/4 shade

If you don't mind an aggressive spreader, vinca/periwinkle is a great groundcover, but mine crawls over barriers into the lawn.

Snow in Summer always looked pretty bad for me after flowering, even when I trimmed it back.

I love pink knotweed. It's always covered in little pink buttons and does well in the heat here in Sacramento. I have it in full, hot sun and it stays beautiful all through summer.

It can spread quickly, but with regular trimming it hasn't choked anything out in my beds.