The method of the present invention first provides a plurality of representative color points existing on the surface of a gamut (s20), and sets a plurality of boundary lines that connect the plurality of representative color points substantially along the surface of the gamut (s22). The method subsequently defines a plurality of unit faces that are respectively surrounded by at least three boundary lines and arranged substantially along the surface of the gamut (s24), and sets a color plane to cross the gamut (s26). The method successively selects cross boundary lines that cross the color plane among the plurality of boundary lines, determines intersections of the cross boundary lines and the color plane, selects unit faces that cross the color plane based on the selected cross boundary lines, specifies cross lines of the selected unit faces and the color plane by connecting the intersections, and joins the cross lines to extract a contour of a cross plane area obtained by causing the color plane to cross the gamut (s28). This structure enables the contour of a cross plane area, which is obtained by causing a color plane to cross the gamut, to be accurately extracted even when the color plane is arranged at any arbitrary position and in any arbitrary orientation.