All-trans 4-keto-retinal has been incorporated into bleached apomembranes of D85N mutant bacteriorhodopsin. The resulting pigment exhibits a pH-dependent equilibrium between forms with a protonated (pink) and an unprotonated (yellow) Schiff base, with pK = 6.15. Optical absorption changes produced by flash illumination of suspensions of the protonated and unprotonated forms have been recorded. Applying an electric field across a thin, dry film of the pink form of the pigment produces spectral changes characteristic of Schiff-base deprotonation. These experiments demonstrate that electric-field-induced Schiff-base deprotonation, first discovered in blue D85N bacteriorhodopsin, can be the basis of electrochromic materials of different colors made using retinal-analog substitution as well as genetic mutation.