Quinacridone is widely used for paint, ink, resin, and fiber as a red pigment. And its color is able to being changed by introducing the methyl- or chloro-substituents. In the industrial process, it is known that the color of the reacting solution changes to blue on adding base, though the depositing pigment is red. In this study, various solutions of sulfo-substituted quinacridone were examined to study the metachromasis of quinacridone, and the mechanism of the metachromasis was discussed.