1. An image processing device comprising: a scanning unit that scans an original document; a color extracting unit that extracts an arbitrary color area in a chromaticity plane with respect to the original document scanned by the scanning unit; a coordinate converting unit that performs coordinate conversion in the chromaticity plane, on the color area extracted by the color extracting unit; and a masking unit that determines a mask area to mask, in accordance with a converted color area on which the coordinate converting unit performed the coordinate conversion.