This paper concerns color measurement and comparison for the purpose of color control in manufacture. Attaching numbers to an inherently subjective sensation (color) has certain difficulties. It is believed, however, that measuring the reflection or transmission properties of a material will yield data that are in direct relation to the color sensations experienced by a subjective observer. Color data are three dimensional, and there does not yet exist a reliable ruler of color difference. Steps toward such measurements are discussed. A cylinder is adopted as a 3-dimensional model of color space, with the dimensions expressing lightness, hue, and chroma. Geometrical distance within the 3-d space should equal perceived color differences. The Munsell color system is one model of this color space. Mathematically, distances within the space are euclidean distances, i.e., the square root of the sum of squares of the component differences. The CIE system record three values that uniquely represent colors. However, this 3-d space does not correspond with the perceptual space defined earlier, and all the variables affecting the transformation (e.g., illumination, size of samples, etc.) are not well understood. One transformation is the L*a* b* formula, proposed in 1976 by CIE. Color differences in the resulting space can be determined directly and objectively by euclidean distance calculation. Problems with this system are a current research topic. Within the current system, tolerances must be agreed upon; some discussion of this problem is provided. An appendix gives formulas for L*, a*, and b* when tristimulus of a colored object are known.