1. An image forming apparatus comprising: a determination unit configured to determine whether input image data has a color space based on subtractive color mixing; a first color conversion unit configured to convert non-subtractive image data into subtractive image data, wherein the non-subtractive image data has a color space based on mixing other than subtractive color mixing, and wherein the subtractive image data has a color space based on subtractive color mixing; an overprint detection unit configured to detect whether overprinting is specified for the subtractive image data; a drawing processing unit configured to perform overprint processing for the subtractive image data if: (1) a color value of a foreground object corresponding to the image data, for which it is detected that overprinting is specified, indicates that a region where the foreground object overlaps a background object is colored, and (2) an underlying region of the background object behind the foreground object is colored, wherein a color value of an overlapping region is overwritten with the color value of the foreground object, and wherein the color value of the overlapping region is not overwritten with the color value of the foreground object if the color value of the foreground object indicates that the region where the foreground object overlaps the background object is colorless; and a second color conversion unit that is configured to convert image data subsequent to the overprint processing into image data having output colors in accordance with characteristics of the image forming apparatus.