After Step S240 is ended, the above-mentioned union is created (Step S250). Then, for the reference color, prediction values of unknown spectral reflectances among the spectral reflectances of the gradation values included in the union are obtained (Step S260). The prediction values of the unknown spectral reflectances are obtained, for example, by performing spline interpolation based on known spectral reflectances for the reference color. In the example illustrated in FIG. 16, the gradation value of 0.3 is included in the union, but spectral reflectances of the gradation value of 0.3 are not obtained regarding the reference color. Hence, in Step S260, the spectral reflectances of the gradation value of 0.3 for the reference color are obtained, for example, by performing spline interpolation based on spectral reflectances of at least three gradation values (at least three of the gradation values of 0, 0.2, 0.5, 0.8, and 1) for the reference color. Incidentally, for example, spectral reflectances eleven gradation values in increments of 0.1 from the gradation value of 0 to the gradation value of 1 are obtained regarding the reference color, and the gradation values for which the spectral reflectances are obtained regarding the prediction target color are 0, 0.5, and 1, then as seen from FIG. 18, spectral reflectances of all the gradation values included in the union are obtained regarding the reference color. In such a case, there are no spectral reflectances obtained in Step S260.
