1. A method of predicting colors of a print produced by a printing press, comprising the step of: with a calculating device predicted colorimetric values of a solid print to be produced on a target printing base with color materials, based on a spectral reflectance of said target printing base and property values of said color materials, said solid print having a combination of area ratios of 0% and 100%; determining differences between measured colorimetric values of a solid color chart produced on a standard printing base with said color materials and having a combination of area ratios of 0% and 100%, and said predicted colorimetric values; and predicting colors of a print to be produced on said target printing base with said color materials and having an arbitrary area ratio, based on said differences.