Threshold color difference ellipses, as represented by the MacAdam ellipses have been compared to unit contours implicit in the CIE94 color difference formula, found to represent small supra-threshold color differences well, both in the CIE chromaticity diagram as well as the a*, b* diagram. The results indicate that the two types of contours are significantly different. While threshold differences are well described by incremented changes in cone sensitivity supra-threshold differences are well-described by increments in the opponent color parameters a*, and b*. A comparison of the hue angles in the a*, b* diagram of the MacAdam ellipses and the CIE94 contours for the same 25 colors indicates no correlation (correlation coefficient -0.1). The conclusion is that threshold and supra-threshold color difference perception operates at different levels in the color vision system.