1. An image forming apparatus comprising: a scanner to scan a document having a plurality of sheets and to generate one image for each sheet of the document; a processor to determine whether the one generated image for each sheet of the document is a color image or a monochrome image; and a printer to perform a color printing of each sheet having the image which is determined to be the color image, and to perform a monochrome printing of each sheet having the image which is determined to be the monochrome image, wherein the processor divides each generated image into a predetermined number of blocks, calculates an average of color signals of pixels in each of the blocks having one or more color signals, compares the calculated average with a predetermined threshold to classify the blocks into one of color blocks and monochrome blocks based on whether the calculated average is greater than or less than the predetermined threshold, and determines whether each image is the color image or the monochrome image using a proportion of the classified color blocks.