A component of a color printing system, such as lookup tables for converting requested marking-agent coverages to corresponding half-tone screen sizes, is calibrated. The calibration can be effected off an arbitrary image and involves making reflectance spectrum measures of the printed image at multiple locations. Expected reflectance measures for each measurement location are computed based on the requested coverage value for each of the marking agents printed there. The computed measures may be dependent on a parameter. A value of the parameter is determined that minimizes the aggregate differences between the sensed and expected reflectance-spectrum measures, and this value is then used in calibrating the component.