An annual flower garden is a fun place to play with color, mainly because you can change it each year. Annuals die back at the end of each growing season, and need to be replaced every spring. Color schemes can be warm, cool, or a combination of both. Warm colors are red, yellow, and orange.
Cool colors are blue, purple, and green.
I use a little white with any combination. Basic garden design mandates taller plants in the back of a rectangular bed, or in the center, if it's a circular bed. Short plants are put in the front for the border, and medium height plants go in between. Color needs to be spread throughout the garden. Don't restrict yellow to the back, red to the middle, and white to the front.