Native plants for color and variety
A customer who has lots of coneflowers and coreopsis in his native plant garden recently asked me for a list of less common wildflowers that would add color to his summer garden, and I worked up this list: Asclepias tuberosa (shown above) -- this is the midsummer blooming orange butterfly weed that goes well with coneflowers and coreopsis
Amsonia -- there are several native versions of this plant, commonly called bluestar. Amsonia illustris or Amsonia tabernaemontana are two that come to mind. Amsonia bloom right alongside and among the coreopsis and spreads nicely among other plants.
Monarda bradburiana (above) is a beautiful and less common native beebalm if you can get it. The color of the leaves is a darker almost smoky green and it forms a nice clump.
Indian physic, Porteranthus stipulatus is a delicate plant with feathery leaves that forms a nice, airy clump among your grasses and has small starry white flowers. It's captivating among other wildflowers.
Also plant some native asters for fall color. Sky blue aster, Symphyotrychium oolentangiense is a nice, delicate blue but can spread quite annoyingly if conditions are right. New England Aster, Symphyotrychium novae-angliae or smooth blue aster Symphyotrychium laeve are also options.