1. An image color discriminating apparatus that discriminates a color of an object image, comprising: a processor comprising: a unit discriminating section that discriminates whether unit data showing a color of a part of the object image corresponds to any position on a two-dimensional color plane; a distribution value acquiring section that acquires a distribution value showing a width of a distribution range of the unit data in the color plane, based on a discrimination result obtained by the unit discriminating section; an image discriminating section that discriminates based on the distribution value whether the object image corresponds to a full color image having mixed color expression, a specific color image not having mixed color expression or an achromatic color image of achromatic color; a lightness histogram generator that generates a histogram of a parameter of brightness, based on a color shown by the unit data; and a discriminating section that discriminates whether the object image corresponds to a gray-scale image having tone expression or a monochromatic image not having tone expression, based on the histogram of the parameter of brightness, when the object image is discriminated to be the achromatic color image.