1. An image processing apparatus having a plurality of image processing modes and outputting images of a plurality of pages on a single page in a combined manner, the image processing apparatus comprising: an image determining unit that determines whether each of the images is a color image or a monochrome image; a character identifying unit that identifies a black character area in each of the images; a color-conversion processing unit that performs either one of a color conversion and a monochrome conversion on each of the images and a monochrome conversion on the black character area; a mode identifying unit that identifies a current image processing mode; a control unit that, when the image determining unit determines that the images include a color image, controls the color-conversion processing unit to perform the color conversion on the images based on a result of determination by the image determining unit and the current image processing mode; and an image output unit that combines the images on which the color conversion is performed by the color-conversion processing unit on a single page and outputs combined images.