Dye penetration into the amorphous parts of plastics was investigated by contacting agar gel with dye solution. The color values, a* and b*, of the agar gel that were penetrated by the dye solution were measured using a spectrophotometer. The concentration of dye in the agar gel was determined from the relationship of dye concentration vs. color values. The penetration amount of dye decreased with increasing concentration of the agar gel, and increased with increasing concentration of the dye solution. In addition, the penetration depth of dye increased with increasing contact time between the agar gel and dye solution. In conclusion, the factors affecting dye penetration in plastics were determined via spectrophotometry using agar gel as a model for the amorphous parts of polymers.