Complementary color schemes, unlike other color schemes typically only have two colors. The base color and it's opposite on the color wheel. Photoshop makes it easy to get the complementary color - simply select the area of color that you want the complement of and hit Ctrl-I. Photoshop will invert it for you. Make sure to do this on a duplicate layer, so you don't lose your base color.
- left - the base color #ffe500
- right - the complement #001aff
Complementary color schemes are often a lot more striking than other color schemes, so use them with care. They are most often used on pieces that need to stand out.