The present invention relates to a photochromic material comprising a proteorhodopsin apoprotein and a retinal analog. In one embodiment, the retinal analog is an azulenic retinoid compound. In another embodiment, the retinal analog is a compound that is structurally similarly to all-trans-retinal. The proteorhodopsin apoprotein and the retinal analog form a photochromic material having different spectral properties from those of a corresponding photochromic material formed by the same proteorhodopsin apoprotein and all-trans-retinal. In one embodiment of the application, the retinal analog-containing proteorhodopsin has an absorbance spectrum that does not overlap significantly with that of all-trans-retinal-containing proteorhodopsin. In another embodiment of the application, the retinal analog-containing proteorhodopsin yields a red-shifted visual chromophore compared with the all-trans-retinal-containing proteorhodopsin chromophore. The photochromic material of the present invention is useful as an optical data storage carrier, a fraud-proof optical data carrier, security ink, and in other optical applications.