In an ink jet printing system consisting of dye-based liquid inks and office copy paper (plain paper), ink penetration has pro-found effects on color rendition. In this report we present methodologies for evaluating these effects by means of experimental data analysis and simulations. On the experimental side, color coordinates of patches printed on two types of substrates, one with ink penetration and another without, have been compared. It is observed that ink penetration has a significant effect on the chroma and hue of the printed colors. On the simulation side, pure effects of ink penetration are studied because ink penetration can be switched on or off at will in the simulation. It is found that ink penetration increases the reflectivity in the absorption band of the ink, while it reduces the reflectivity in the transparent region of the spectrum. This explains the experimental observation that saturation of color is reduced and the hue of the color is changed upon ink penetration. Additionally, advantages and disadvantages of the evaluation methods have been discussed.