1. A system comprising: a controller operable to acquire measured reflectance values for a blank portion of a print medium, to analyze the measured reflectance values for the blank portion to determine if optical brighteners have been added to the print medium, and to acquire measured reflectance values for colorant applied to the print medium by a printer, wherein the controller is further operable, if it determines that optical brighteners have been added to the print medium, to reduce the measured reflectance values for the colorant, and to initiate a calibration for the printer using the reduced reflectance values for the colorant, wherein the controller is further operable, if it determines that optical brighteners have not been added to the print medium, to initiate the calibration for the printer using the originally measured reflectance values for the colorant, wherein the calibration defines amounts of colorant to add to the print medium in order to generate particular colors on the print medium, wherein the controller is further operable to reduce a reflectance value for the colorant applied to the print medium at each of multiple wavelengths by: acquiring a measured reflectance value (R) of the print medium for the wavelength; comparing R to a predefined first threshold value (T 1 ); responsive to determining that R is larger than T 1 , reducing the measured reflectance value for the colorant in the wavelength by a first factor (F 1 ); responsive to determining that R is not larger than T 1 , comparing R to a predefined second threshold value (T 2 ); and responsive to determining that R is greater than T 2 , reducing the measured reflectance value for the colorant in the wavelength by a second factor (F 2 ).