In order to enlarge a gamut in the low-brightness region of the hue of a special color, an image forming apparatus comprises an input unit for inputting color image data for which an image is to be formed; a color separation unit for generating color data in which the color image data has been separated into each of the plurality of colors; and an image forming unit for forming a color image based upon the color data; wherein said color separation unit generates the color data so that the color data has a region in which amount of the color material of the color black used is increased and amount of the color material of the special color used is increased as brightness declines in a low-brightness region in the hue of the special color.