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, wherein when the current image processing mode is a mode based on a picture area, granularity, glossiness, color reproducibility, or high-density-area reproducibility, the control unit controls the color-conversion processing unit to perform the color conversion on the images, other than the black character area in each of the images, regardless of the result of determination by the image determining unit.