Color needs to be spread throughout the garden. Don't restrict yellow to the back, red to the middle, and white to the front. Each height needs each color that you have chosen. Say you choose a cool color scheme - blue, purple, green, and white. A blue morning glory could even climb above the garden.
Green coleus is an effective mid-height foliage plant.
An appropriate border can consist of violet pansies, white alyssum, and deep blue lobelia. A warm color scheme would include red, yellow and orange flowers. White always helps the intensity of such a plan. I love tall red and yellow snapdragons, yellow and orange calendula, and white bachelor buttons. I love a blue and white garden.