Abstract: A colorant-amount transforming method for transforming a first colorant amount for forming an image on a first medium into a second colorant amount for forming the image on a second medium whose colorimetric value differs from that of the first medium. The first colorant amount is transformed into a colorimetric value by a first transformation for transforming the first colorant amount into a colorimetric value corresponding to a tristimulus value (or a spectral reflectance) in which a portion of contribution of the tristimulus value (spectral reflectance) of the first medium itself to the tristimulus value (spectral reflectance) of the image formed on the first medium with the first colorant amount is replaced by the tristimulus value (spectral reflectance) of the second medium itself in accordance with a contribution rate.