Tri-stimulus values XYZ express the quantities of basic stimulus of color, however, these are difficult to use for color control in business enterprises. Therefore, for practical use, they are converted into a specification method corresponding to the three attributes of hue, lightness, and chroma. A space intended so that a color difference perceived as an equal dimension may correspond to an equal distance in the color space is called USC, and the dimension of color difference is expressed by the distance on this UCS coordinate.

Like this, it is convenient for color control to convert a coordinate of tri-stimulus values to a coordinate that corresponds to color sense. Method of conversion are forth in color exprrssion method, among which the most widely utilized is the L*a*b* color specification system. The defining equation of conversion is omitted here. The figure of a sphere on the left is what specifies color on the three-dimensional coordinate. And the L*a*b* color specification system chromaticity diagram (hue and chroma) is what expresses color on a horizontal section of a certain lightness. Therefore, the achromatic coordinate of that lightness is projected near the origin of a*=0, b*=0. With the center here, the color increases its lightness in spreading outwards. As seen above, a color coordinate that corresponds to perceived color is utilized for color control.