A secondary color is used for printing an image with a printer so as to reduce the granularity of the printed image, particularly the light image area. Data of magenta and cyan color components of image data is subjected to under-removal processing to generate blue ink image data as the mentioned secondary color. A blue ink dot is printed in accordance with the blue ink data obtained by the under removal processing to replace the magenta and cyan data. As a result, it is possible to reduce the granularity that would otherwise be caused by localized distributions of magenta and cyan dots.