A first color conversion unit converts non-subtractive image data into subtractive image data. An overprint detection unit detects whether overprinting is specified for the subtractive image data. A drawing processing unit performs overprint processing for the subtractive image data if a color value of a foreground object, for which overprinting is specified, indicates that a region where the foreground object overlaps a background object is colored, and an underlying region of the background object behind the foreground object is colored. The color value of an overlapping region is overwritten with the color value of the foreground object. The overlapping region is not overwritten if the region where the foreground object overlaps the background object is colorless. A second color conversion unit converts image data subsequent to the overprint processing into image data having output colors in accordance with characteristics of the image forming apparatus.