The color (RGB value, that is, Pix0 to Pix255 in FIG. 7) of the color component image shown in FIG. 7 is determined by combining the index value (corresponding to the color number) indicated as each pixel of the component image shown in FIG. 6 and the entry of the display color corresponding to the color number included in the color palette shown in FIG. 5. For example, index value 0 of a pixel in the component image data shown in FIG. 6 is replaced by RGB (0, 0, 0) that is the display color data corresponding to color number 0 in the color palette shown in FIG. 5, and index value k (k=0, 1, . . . , 255) of a pixel in the component image data shown in FIG. 6 is replaced by RGB (rk, gk, bk) that is the display color data corresponding to color number k in the color palette shown in FIG. 5. That is, if the display color data of the color palette are in the red range, the color component image data becomes the data of the red range; if the display color data of the color palette are in the blue range, the color component image data becomes the data of the blue range; and if the display color data of the color palette are in the green range, the color component image data becomes the data of the green range.