While a little bit of this and a little bit of that in a flowerbed can create a charming, eclectic look, sometimes it helps to consider the effect you're going for before you go plant shopping. Color and color combinations can be playful or sophisticated, relaxing or energizing, traditional or whimsical.
Monochromatic Color Schemes
A monochromatic color scheme uses the same color for all flowers. The subtle differences in shades and hues adds interest to this color scheme. The size and shape of the plants will stand out with a monochromatic scheme. You can create a relaxed, sophisticated look by using all white flowers. This is known as a moon garden because white is especially noticeable at night. Include the moonflower (Ipomoea alba), which grows as a perennial in U.S. Department of Agriculture plant hardiness zones 9 through 11. Or you can go bold with all red. Red attracts hummingbirds and butterflies, so this will be a lively garden in fact as well as in appearance. Consider red canna lilies (Canna generalis), which are hardy in USDA zones 8 through 12 but will grow in cooler climates if the bulbs are dug up each year.
Analogous Color Scheme
For an analogous color scheme, choose two or more colors that are next to each other on a color wheel. Examples of this color scheme are yellow, yellow-green and green or purple and lavender. This is a harmonic, peaceful color scheme that's easy on the eyes. For example, plant sweet Williams (Phlox divaricata), choosing reddish-purple, purple and lavender varieties. They grow in USDA zones 3 through 9.
Complementary Color Scheme
A complementary color scheme uses two colors that are opposite each other on the color wheel, such as purple and yellow. Complementary color schemes really pop because both colors appear more vibrant when placed next to each other. This is an exciting color scheme but can also be sophisticated, depending on which flowers you choose. Pair golden-yellow black-eyed Susans (Rudbeckia hirta) with purple coneflowers (Echinacea purpurea). Black-eyed Susans grow in USDA zones 3 through 7, and purple coneflowers grow in USDA zones 4 through 10.
Polychromatic Color Scheme
A polychromatic color scheme uses a mix of colors. A kaleidoscope of color can be fun and playful, but can also look chaotic if you're not careful about plant placement. A triad scheme uses three colors of equal distance on the color wheel, such as red, blue and yellow, or orange, purple and green. The effect is energetic but balanced. Try orange dahlias (Dahlia spp.) with purple bellflowers (Campanula carpatica). Dahlias grow in USDA zones 8 through 10 but can be grown as annuals in warmer zones, and bellflowers are hardy in USDA zones 4 through 8.