This study aims at modeling ink spreading in order to improve the prediction of the reflection spectra of three ink color prints. Ink spreading is a kind of dot gain which causes significant color deviations in ink jet printing. We have developed an ink spreading model which requires the consideration of only a limited number of cases. Using a combinatorial approach based on Pólya's counting theory, we determine a small set of ink drop configurations which allows us to deduce the ink spreading in all other cases. This improves the estimation of the area covered by each ink combination that is crucial in color prediction models. In a previous study, we developed a unified color prediction model. This model, augmented by the ink spreading model, predicts accurately the reflection spectra of halftoned samples printed on various ink jet printers. For each printer, the reflection spectra of 125 samples uniformly distributed in the CMY color cube were computed. The average prediction error between measured and predicted spectra is about ΔE = 2.5 in CIELAB. Such a model simplifies the calibration of ink jet printers, as well as their recalibrations when ink or paper is changed.