A CPU 20 assumes a virtual color monitor in which actual chromaticity coordinates of colors expressed by fluorescent materials are replaced by virtual chromaticity coordinates having the same hues as those of the actual chromaticity coordinates but higher saturations than those of the actual chromaticity coordinates. The CPU 20 converts colorimetric values X, Y, and Z into luminance-linear values r′, g′, and b′ for red, green, and blue, based on a relationship of color conversion for the virtual color monitor at step S23. The CPU 20 then sets any one of the values r′, g′, and b′ to L at step S24 and compares the value L with i at step S26, where i is the &ggr;-th power of j and greater than 0. In case that the value L is less than i, the CPU 20 calculates a value c based on the remaining values other than the value L among the values r′, g′, and b′ at step S27. When −ck≦L