The Yule–Nielsen modified spectral Neugebauer model (YNSN) enhanced for accounting for ink spreading in the different ink superposition conditions requires a spectrophotometer to measure the reflectances of halftone calibration patches in order to compute the ink spreading curves mapping nominal ink surface coverage to effective ink surface coverage. Instead, as a first step towards the "on the fly characterization of printers, we try to deduce the ink spreading curves from digitized RGB images of halftone calibration patches. By applying the Yule–Nielsen broadband formula to the average RGB intensities, we deduce the effective dot surface coverages of halftone calibration patches. We then establish the ink spreading curves mapping nominal dot surface coverages to effective dot surface coverages. By weighting the contributions of the different ink spreading curves according to the ratios of colorant surface coverages, we predict according to the YNSN model the reflectance spectra of the test patches. We compare the prediction accuracy of the YNSN model calibrated by digitized RGB intensities of the calibration patches with the prediction accuracy of the same model calibrated by the spectral reflectances of these calibration patches. The decrease in spectral prediction accuracy due to RGB model calibration is about 30% for ink jet prints and 10% for thermal transfer prints.