1. An image processing method of processing image data obtained by, in an image reading apparatus, causing light emitting means for emitting light components of three primary colors to irradiate an image original with the emitted light components and causing light receiving means to receive the reflected light, the method comprising: a reading step of, in the image reading apparatus, performing primary color reading by causing the light emitting means to sequentially irradiate a printing medium without any printed image with the light components of three primary colors and causing the light receiving means to receive the light reflected by the printing medium, and performing complementary color reading by causing the light emitting means to simultaneously irradiate the printing medium with one of three different color combinations, which includes two of the light components of three primary colors, and causing the light receiving means to receive the light reflected by the printing medium; a conversion step of converting image data generated by the complementary color reading into image data represented by a color space that is the same as that representing image data generated by the primary color reading; a generation step of generating a brightness signal distribution of each color component from each of image data generated by the primary color reading and the complementary color reading in the reading step and the conversion step; a selection step of selecting a lowest one of signal levels from each of the brightness signal distributions generated in the generation step; a forming step of forming conversion tables, respectively corresponding to the primary color reading and the complementary color reading, which are used for converting the image data so as to make a pixel value higher than a pixel value corresponding to the lowest one of the signal levels selected in the selection step close to a pixel value representing white; and an image processing step of performing under color removal of the image data using one of the conversion tables in accordance with a result of whether the reading of the image original is the primary color reading or the complementary color reading.