Summertime flowers that can take the sun and heat tend to have dusty colors and silvery leaves. Not all, but many.
Wormwood is a good example of summertime beauty with cool, silvery, water-surface coloring. The drier the better. Here, the groundcover is gravel. Artimisia has been widely used as a medicinal herb, too. Sweet Annie can be started from seed in the spring.
Agastache has come into its own with more colors than the familiar blues.
Sunset Hyssop is sown from seed in the spring and blooms heavily in August's heat.
Shades of Orange Agastache is available from High Country Gardens.
Seseli elatum grown by Tom Clothier and gummiferum at Annie's Annuals are both August winners. Clothier's seed germination database says to sow seeds at 68-degrees and if they don't germinate in a month, chill them and try again.
With all varieties of Oregano available, a gardener would be hard pressed to say there isn't a flower color to go with their scheme. This thorough article by
Sally Bernstein can help you choose which seeds or plants to put in next spring. Pantry Garden Herbs has seeds and seed starting help.