The eye's ability to separate two distinct colors depends of the hue, saturation and brightness of the observed color. As a consequence, the sensitivity to color differences will vary from one region to another within the CIExy diagram. Many studies have tried to study these variations: Mac Adam [] defined 25 points, spread out inside the (x,y) diagram, around which he drew ellipses with different sizes and orientations: all the colors inside an ellipse appear to be matched with the central color of the ellipse for a standard observer