Complementary colors are the colors that are opposite from each other on the color wheel:
Red and Green
Orange and Blue
Yellow and Purple
Using complementary colors in a piece of artwork makes the colors stand out more, and almost always makes a good piece of art great. Hannah Schmitt, age 10, used complementary colors in her illustrations for At the Magical Fountain. Look at how she used bits of bright pink (pink is really light red!) to complement her greens and make the artwork interesting.