Color is a 5-dimensional perception that is not only related to the spectrum coming from an object, but also strongly related to its spatio-temporal context. It is a powerful feature that allows humans to make reliable inferences about objects that would be nice to understand and mimic in artificial vision. In this course, we derive the linear CIE tristimulus theory from its experimental color matching foundations. We derive the relations between spectrum and tristimulus vectors through the color matching functions, the chromatic coordinates, chromatic purity and luminance. Phenomenology of color discrimination and adaptation reveals the limitations of the linear description and sets the foundations of color appearance models. In addition, we link the above perceptual representations of color with the conventional representation of color in computers.