A photonic crystal structure with a refractive index that gradually changes in the specific direction of the crystal is demonstrated (see Figure). The colloidal photonic crystal was obtained by infiltration of a monomer and organic dopant with a high refractive index into the colloidal opal, followed by interfacial gel polymerization. The resulting colloidal crystal has a gradually varying stop-band at different positions in the crystal when the incident light is normal to the {111} crystallographic axis.