1. An image processing device that generates double color image data for formation of a double color image on a basis of multicolor image data which is image data of a multicolor image, the double color image being expressed by (i) a black color material and (ii) a single chromatic color material or a combination of a plurality of chromatic color materials and being composed of black and other single color, the image processing device comprising: a saturation calculating section that calculates saturation of each pixel on a basis of the multicolor image data; a pixel determining section that determines a pixel as a chromatic pixel if the saturation of the pixel having been calculated by the saturation calculating section is not less than a preset threshold value, and determines a pixel as an achromatic pixel if the saturation of the pixel having been calculated by the saturation calculating section is less than the threshold value; an achromatic color data generating section that generates pixel data for formation of a black image, for the pixel having been determined as the achromatic pixel by the pixel determining section; a chromatic color data generating section that generates pixel data containing single color image data for formation of an image of the single color, for the pixel having been determined as the chromatic pixel by the pixel determining section; and a pixel data synthesizing section that generate the double color image data by combining the pixel data having been generated for the achromatic pixel by the achromatic color data generating section and the pixel data having been generated for the chromatic pixel by the chromatic color data generating section, the chromatic color data generating section generating, for the chromatic pixel, the single color image data such that a density of the single color increases with increase in the saturation of the chromatic pixel having been calculated by the saturation calculating section.