Complementary colors are colors that sit on the opposite side of the color wheel from each other.

This means a lot of things:

  1. They fulfill each other in a color sense.  That is, one completes the areas of the color spectrum that the other misses.
  2. When combined they form a grey, or near black, or brown.
  3. There is an attraction between the colors – more on this in the section of movement with complementary colors.