We introduced dyes, the colors of which are variable by changing the pH of their medium solutions, into gelled colloidal photonic crystals (GCPCs) and investigated their reversible pH-responsive optical behaviors. When the extinction position of the dye overlaps with the Bragg peak of the GCPC, the Bragg-diffracted light can be trapped in the GCPC; when the extinction position of the dye is shifted out of the Bragg peak of the GCPC, the Bragg diffracted light is free and can escape out of the GCPC. The optical behaviors of dye/gelled colloidal photonic crystal composites are reversible to the pH changes of their surrounding medium.