When color materials of more than four colors are used, a signal for the amount of toner used within a range appropriate for the apparatus and toner characteristic is generated using a color separation table. However, a printer receives print data of various formats, such as image data separated into signal values of more than four colors, or image data of a special format to which a six-color separation table cannot be applied. Hence, an image signal representing a combination of colors is input. The sum of the signal values of colors in each pixel of the image signal is calculated and compared with a limit value. When the sum exceeds the limit value, the signal values of base colors are replaced with that of a spot color based on a replacement table for replacing the signal values of the base colors with that of the spot color.