Look into Daylilies. There are re-bloomers & colorful "everbloomers" ... that bloom all summer & in the fall season too. .. Stella D' Oro, Happy Returns, Pardon Me, & Rosy Returns,the first PINK everblooming Daylily.
Right now you can plant potted plants that are blooming like Bee Balm, Butterfly Bush (buddleia ), Coreopsis, Delphinum, Verbena, Echinacea (Coneflower), Blue sage, Salvia, Dianthus , Maltese Cross, Honesty (Lunaria), Geraniums, Lavender, Daylilies, Gaillardia, Black-Eyed Susan, Shasta daisy, Russian Sage & Veronica (Speedwell) & Hydranga.
The Knockout roses should do well throughout the summer & until frost too( if your area has frost). There's three diffferent varieties of Knockout roses & at least 3 varieties of Echinacea too
Feast your eyes on these perennials:
http://www.gardenmob.com/blog1/category/on-perennials/the-great-perennials/
Good luck! The flower centers, Home Depot & Lowe's are starting to offer deep discounts now. I just bought a potted Lavender at 1/2 price.