1. A color determining apparatus which determines whether an document is a color document or a monochrome document based on image data of the document including a plurality of pixel data, comprising: a pixel color determination part which performs color determination of pixels based on a luminance component value and a color component value of the pixel data; a pixel color re-determination part which set one of the pixels determined to be a color pixel by the pixel color determination part a target pixel in turn, and at a time when a plurality of filters having different outline shapes are applied to the target pixel, performs color re-determination of the target pixel according to whether all of the pixels included in the filter are determined to be color pixels or not by the pixel color determination part; and a first document color determination part that determines whether the document is color or monochrome by comparison of a number of the pixels determined to be color pixels by the pixel color re-determination part and a reference value set in advance.