1. A color determination device, comprising: a region determining means for determining whether each of image data expressing a color of each portion of a determining image belongs to any one of a plurality of partial regions obtained by dividing a two-dimensional color plane with prescribed color determination boundary lines; means for counting a region determination result of the region determining means; and a color determining means for determining a color of the determining image in accordance with a count result of the means for counting; wherein a gradient of the color determination boundary lines with respect to a coordinate axis of the two-dimensional color plane is a rational number.