The color rule still applies. Choose similar flower types and similar colors. Contrasting border plants help the color scheme stand out from lawn and trees. An excellent plant for this is dwarf Dusty Miller with its gray- silver leaves. Another would be Irish Lace marigold with its finely cut mint-green leaves.
White alyssum is good because it adds a mass of white.
Use the skinny egg- shape planting diagram for these, rather than planting in a narrow band that precisely follows the shape of the bed. Plant breeders have greatly improved the range of colors and shades in many popular annuals. Here's a partial list of annuals that are available as seeds or transplants. Red to pink flowers. Snapdragon, aster, begonia, celosia, dahlia, sweet William, gazania, strawflower, stock, nicotiana, petunia, portulaca, nasturtium.