1. An image processing apparatus comprising: a first medium colorimetric value acquisition unit configured to acquire a first medium colorimetric value obtained by measuring a medium containing a fluorescent whitening agent at a first temperature; a second medium colorimetric value acquisition unit configured to acquire a second medium colorimetric value obtained by measuring the medium at a second temperature; a patch colorimetric value acquisition unit, configured to acquire reference patch colorimetric values obtained by measuring patches of a plurality of colors formed on the medium at the second temperature; target temperature acquisition unit, configured to acquire a target temperature; medium colorimetric value estimation unit, configured to estimate, based on the first and second medium colorimetric values, a third medium colorimetric value obtained when the medium is measured at the target temperature; and patch colorimetric value estimation unit, configured to estimate, based on the second and third medium colorimetric values and the reference patch colorimetric values, patch colorimetric values obtained when the patches of the plurality of colors formed on the medium are measured at the target temperature.