1. A document processing apparatus comprising: a determination unit that determines at least some of a plurality of colors contained in input document data as a determined color group; a retrieval unit that calculates an angle between (i) a line, in a given color component space, connecting a given point, which is defined in connection with color blindness of a human being, and one of the colors of the determined color group and (ii) a line, in the given color component space, connecting the given point and each of the other colors of the determined color group, and determines as to whether or not each calculated angle is less than a given threshold value, wherein if the retrieval unit determines that one calculated angle is less than the given threshold value, the retrieval unit determines said one of the colors and said other of the colors, which corresponds to the one calculated angle, as a confusion color set; and a processor that performs a given process for portions, each having any of the colors contained in the confusion color set determined by the retrieval unit, in the input document data.