Dye penetration into the amorphous parts of polymeric materials was investigated by contacting agar gel with a dye solution. Three dyes with similar chemical structures but different molecular sizes and substituents were used to evaluate the dye penetration into the agar gel. As a result, in order to determine the amount of dye penetration into the agar gel. It was better to use the color difference (ΔE*_ab) which is calculated by color value (a*, b* and L*) of the agar gel penetrated. The amount of penetration into the agar gel was correlated with the SP value relationship correlation was observed, suggesting that the amount of penetration depends on the solubility parameter based on the chemical structure of the dye.