Colors that create contrast are opposite each other on the color wheel. These colors when put next to each other make things pop. They are in the simplest form primary and secondary colors. Primary colors are: red, blue and yellow. No other colors are mixed to create them they are pure. Secondary colors are: green, orange and purple. Secondary colors are created by mixing primary colors.
Green = blue + yellow
Orange = red + yellow
Purple = red + blue
Hence, complementary colors are: red/green, orange/blue, purple/yellow. The best way to remember? Since secondary colors are two primary colors mixed, the complementary color is the third primary color. If yellow and red make orange then the remaining primary color is blue. Blue is the complementary of orange. See? Easy peasy.