Have you thought about a collection of succulents? They offer a terrific variety of colors and shapes, and they're especially durable during the hottest summer weather. Even in a site with afternoon shade, they'll do well on your porch. They also tolerate a more restricted root growing area than most of the more tender succulent annuals. But speaking of tender succulent annuals, you might have good luck with the Wave Series petunias. They're such vigorous growers and bloomers that you'd probably want to plant just one per pot. They'll be less tolerant of drying out than the succulents are, but they can offer a lot of color and sprawling blooms.