A frame buffer stores pixel data for each pixel. The pixel data are converted into color data according to color data at a corresponding address in a first look-up table (LUT). An overlay memory stores change data representing presence/absence of color change for each pixel and change pixel data. According to contents of change pixel data in the overlay memory, color data at a corresponding address in a second look-up table are read, thereby converting the change pixel data into color data. To display an image, data stored in both of the frame buffer and the overlay memory are read for each pixel. To perform color change, the change pixel data are used instead of the pixel data. When change data in the overlay memory indicate presence of color change, change pixel data are converted into color data with reference to the second look-up table. Then the color data are exhibited on a CRT display. Color change can be also performed by an address designating circuit which is provided in place of the second look-up table and is used for changing an address to be read with reference to the first look-up table.