It is argued that it is the receptive field of visual neurons that enables people to maintain approximately color constancy despite variation in the spectral power distribution of the ambient light and despite variation in the color of nearby objects. A lightness algorithm based on the model function of receptive field of human visual neurons is proposed and is proved to be a general color-constant algorithm as well as an adequate model of human color constancy performance. Problems about lack of color constancy of colorimeters are briefly discussed.