Print data representing a superimposed image is generated by developing the main image on a first memory area, developing an additional image on the first memory area so as to be overlaid on the main image and on a second memory area having the same size of the first memory area. A first color conversion is applied to the image on the first memory for portions not corresponding to the image on the second memory area. A second color conversion, which is independent of the first color conversion, is applied to other portions of the image on the first memory for portions corresponding to the image on the second memory area. The image developed on the first memory area is output as the print data after the first color conversion and the second color conversion are applied.