1. A method of processing an image of an image forming apparatus, the method comprising: dividing an image into a predetermined number of blocks; calculating an average of color signals of pixels in each of the blocks such that each of the pixels has a color signal; comparing the calculated average with a predetermined threshold; classifying the blocks into color blocks and monochrome blocks based on whether the calculated average is greater to or less than the predetermined threshold; and determining whether the image is a color image or a monochrome image according to a proportion of the color blocks.