The display color of, for example, a button image responsive to a command input into a facility operation display device is controlled by a palette value having a smaller number of bits than an RGB value. When the display color of the button image is changed, the palette value of a drawing object associated with the button image is changed to an RGB value. This eliminates the necessity of incorporating, for example, a high-performance CPU as a central arithmetic unit. In addition, it is not necessary to pre-store images corresponding to several kinds of display colors specified by RGB values, to thereby eliminates the necessity of incorporating, for example, a high-capacity storage medium in the facility operation display device. Accordingly, the device cost can be reduced.