How much space do you have in the flower bed? Do you need something for the front of the border or the back?

I always like purple with yellow. I've seen bright yellow and light yellow potentilla. I would base my contrast color on the intensity of it so it doesn't take away from it.

Combining plants is really a personal preference, but I like to see contrasting textures and flowers. Also, in my area potentilla doesn't look like much in the winter. So...it might be nice to add something that would look nice in winter as well.

A red plant would look nice too. Something like barberry. There are many varieties and similar plants of various sizes. Dark evergreens would be another option.

A butterfly bush gets very large so if you go that route remember it will really grow. The flower of this plant are beautiful but may dwarf the small potentellas.

Perennials that come to my mind would be a smaller catmint like walkers low, the larger agastache blue fortune. Daisies, even a cranesbill to climb through the plant, veronica.

Annuals: Petunias would look nice and the assorted colors would work as would a single color. Blue Victoria Salvia or a variety of red.

I hope that gives you some ideas, but really they will blend with anything you love.

I was just thinking an orange flower would be pretty too if you prefer a warmer border.