Colorimetry is concerned with quantitatively studying color, a subjective experience. Not until we can put our experience into numbers can we rigorously study colors. In Sect. 3.3.3, we have seen two ways to geometrically interpret a color as a point in a three-dimensional space: the cone space and the CIE 1931 RGB space. We will study a few other ways to quantitatively analyze colors in this chapter. Before moving on, it is critical to note that classic colorimetry is concerned only with color matching under the same viewing condition. It tells us if two objects or light sources have the same color when viewed under exactly the same conditions (e.g., ambient illumination). It does not tell us (1) how different two colors are and (2) the actual appearance of a color, which depends on the viewing condition. This chapter will focus mostly on the classic colorimetry. Color difference will be briefly discussed in Sect. 4.7; Fairchild (Color appearance models, Wiley, 2013) is a classic reference on color appearance modeling, which we will also briefly touch upon in Sect. 4.6.3.