1. An image forming apparatus capable of forming an image on a recording medium on the basis of image data with use of recording agents of either two or more than two colors, comprising: a detecting section for detecting, out of an image represented by the image data, a first region and a second region, of which the first region is assigned first color data indicating a predetermined color so that it can be formed solely of one of the recording agents that corresponds to the predetermined one color, and the second region, which is arranged adjacent to the first region, is assigned second color data indicating a color different from the predetermined color so that it can be formed of a recording agent different from the recording agent for forming the first region; a region adjusting section for specifying, on the basis of a detection result produced by the detecting section, a third region which is continuous with the second region and overlaps with a part of a periphery of the first region which is contiguous to the second region, and assigning the third region third color data indicating a color that can be represented by the recording agents used for forming the first and second regions; and an image forming section for forming an image on a recording medium with use of the recording agents corresponding to the colors indicated by the first to third color data assigned to the first to third regions, respectively.