In this paper, a optical pseudocolor encoding method upon the difference between two photographs is presented. We show that the difference between two pictures with continuous change in gray which is discriminated simultaneously by eyes, can be detected by use of two sources of different color. The pseudocolor image from the optical system is processed in select-color and differentiation by video system, thus the signal of difference is enhancad. The theory is agreement with experiment. Finally, some applications are discussed.