1. An image processing apparatus for converting a monochrome image into a color image, the apparatus comprising: an attribute determination portion that determines an attribute of a pixel to be any of three attributes of a text, a graphic, and a photograph, and divides a region in accordance with the attribute of each pixel; a selection portion that selects, for each of the attributes, one colorization method from among a plurality of predetermined colorization methods; a processing portion that colorizes the monochrome image by applying the colorization method selected by the selection portion; and a determination portion that determines whether or not a change of selection of the colorization method is necessary based on an attribute of each pixel of the monochrome image and an attribute of each pixel of a colorized image by the processing portion, or, alternatively, based on a color of a pixel adjacent to a border between regions that is a border between the regions whose attributes are different from each other; wherein, when the determination portion determines that the change of selection is necessary, the selection portion changes selection of the colorization method for at least one attribute such that a combination of the colorization method applied to the monochrome image and the attribute is changed, and the processing portion performs re-colorization on a region having an attribute for which the selection of the colorization method is changed by the selection portion.