In the summer, I love hot colors like red and purple. The juxtaposition with the chartreuse Lady's Mantle really sets apart the hot colors. The key is to go to independent growers, not chain stores. Get to know the small local growers and find out what their specialties are. Don't buy for convenience, buy the specialties of a particular grower.
Farmers markets are also a good place to buy.
I buy a lot from the Germantown Greenhouse because the owner is an impassioned grower. He grows what he thinks is wonderful, he doesn't just grow to sell. I'd like to add Japanese anemone, asters and clematis. I'd also like to introduce vegetables into the garden. I could put them along the house border where they get sun all morning and the brick holds the heat.