A detailed description will be given of how to obtain the first relational equation. As mentioned above, regarding the sample color, the calorimetric values (spectral reflectances) of all the gradation values in increments of 0.1 from the minimum gradation value (gradation value of 0) to the maximum gradation value (gradation value of 1) are obtained. That is, data corresponding to curves (curves which represent the spectral reflectances) as schematically illustrated in a portion A of FIG. 5 are obtained for all of eleven gradation values in increments of 0.1 from 0 to 1 (in FIG. 5, horizontal axes represent wavelength (unit: nm), and vertical axes represent reflectance). Note that the portion A in FIG. 5 illustrates only curves corresponding to four gradation values among the eleven gradation values (the same also applies to a portion B in FIG. 5). A curve denoted by reference symbol 51 is a curve for the minimum gradation value, and a curve denoted by reference symbol 52 is a curve for the maximum gradation value. Such data are subjected to normalization with the spectral reflectances of the minimum gradation value being taken as 1. Thus, data corresponding to curves (curves which represent the spectral reflectances) as schematically illustrated in a portion B in FIG. 5 are obtained. However, a straight line is obtained for the minimum gradation value taken as a reference of the normalization.
