A spectral reflectance, corresponding to multiple quantities of coloring materials, is obtained, and a target spectral reflectance, corresponding to data of a color of interest, calculated based on the spectral reflectance. The target spectral reflectance is calculated in a manner that makes the spectral reflectance change contiguously, across an interval of multiple colors that are reproduced by the quantities of coloring materials in question. A decision of the quantities of coloring materials in question is thus made that corresponds to the target spectral reflectance.