Psychophysical and physiological studies during the last thirty years or so provided enough evidence for the zone theory to be accepted as the general framework for human color vision. Although this theory says that the initial trichromatic signals (i.e., the responses of three classes of cones) are transformed to the color-opponent signals, many problems still remain to be solved. Current knowledge about the mechanisms of color vision has important implications for further research. In this article, I discuss mainly psychophysical and physiological issues regarding the mechanisms of color vision within the framework of the zone theory.