Based on the fact that the human eye has three different types of color sensitive cones, the response of the eye is best described in terms of three ‘‘tristimulus values’’. However , once this is accomplished, it is found that any color can be expressed in terms of the two color coordinates x and y. The colors which can be matched by combining a given set of three primary colors (such as the blue, green and red of a color television screen) are represented on thechromati city diagram by a triangle joining the coordinates for the three colors.