1. A method of controlling output of a display unit, comprising: outputting a background image; calculating a difference between an arbitrary expression value of an arbitrary screen configuration element to be displayed on a given area of the background image and an actual expression value of the given area of the background image, each expression value including at least one of a color and a lightness; selecting an arbitrary expression value for the screen configuration element at the given area; and displaying the screen configuration element at the given area of the background image based on the difference of the expression values.