Combining and contrasting color combos
A monochromatic color scheme uses plants that all flower in the same basic color. All whites, all reds - just make sure that the reds are related - no purpley reds fighting with orangey reds.
Contrasting colors are those opposite each other on the color wheel:
Complementary colors, the ones next to each other on the color wheel also work well:
When in doubt, add white flowers, or plants with silver foliage. These can make the most clashing combinations appear to work. So face it, you can't really mess up, so, enjoy! Oh, and remember, leaves also have color - green (surprise!), purple, silver, bronze, and even blue.