Purple fall-flowering plants?

I am working on creating a flowering garden in front of my patio. It will contain creeping phlox, bearded iris, and coral bells in a blue-purple-black-white color scheme. I want a centerpiece along the back that should be 2-4' tall, 2' deep, and 3-5' wide, and I'm willing to do some maintenance to help it maintain that shape. I'd also like it to bloom in late summer and fall to complement the blooming seasons of the phlox and irises. Does anyone have suggestions?