Color vision starts with light striking the retina, a photosen-sitive organ in the back of the eye. The cells in the retinaencode the light into signals, which are transmitted to the brain. There, the signals are interpreted to provide the perception of color. This chapter focuses on color vision at the encoding level, where it is represented by three signal values generated by the retina. This encoding is the foundation for color measurement, and for the representation of color as a triple of red, green, and blue values.