title: How to create enum method
body: <p>I have this enum : <code>public enum Color { RED, YELLOW; }</code>.
  And I want to create a method that returns the opposite color of the actual one. So if I have RED, my method returns YELLOW. But I don't know the process to follow to do that.
  First step : <code>public Color opposite() { return ??? }</code></p>