1. A color measuring method for determining a spectral reflectance, under a predetermined illumination environment, for a printed matter including a paper containing a fluorescent whitening agent, the method being comprised of: a first step of measuring a spectral reflectance Pt(&lgr;) of said paper under a first illumination environment and a spectral reflectance Pu(&lgr;) of said paper under a second illumination environment; a second step of computing a difference, obtained by subtracting said spectral reflectance Pt(&lgr;) from said spectral reflectance Pu(&lgr;), as a differential component Fp(&lgr;) of said paper due to the fluorescent whitening agent; a third step of measuring a spectral reflectance Rt(&lgr;), under said first illumination environment, for a region to be measured of said printed matter; and a fourth step of performing a correction based on said differential component Fp(&lgr;) on said spectral reflectance Rt(&lgr;) to compute an estimated spectral reflectance, under said second illumination environment, for said region to be measured.