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 determines a set of confusion colors of the colors contained in the determined color group based on confusion color information defined in association with color blindness of a human being in a predetermined color component space; and a processor that performs a predetermined process for portions of the colors contained in the confusion color set determined by the retrieval unit in the input document data.