Based on the psychophysical method of constant stimuli, an experiment was carried out, using CRT-generated stimuli and in CIELAB color space, to test the visual color differences in relation to colorimetric scales. The CIE Gray and Blue color centers, the most basic color and, perceptually, the most different color, respectively, were chosen, and the maximum average size of test color difference was 12 CIELAB ΔE units. The resultant visual data, processed via probit analysis, were used to analyze the relationship between color discrimination threshold and suprathreshold color-difference perception. The equal color-difference contours, corresponding to all the visual scales, 4.0, 8.0, and 12.0 CIELAB ΔE units, were well fitted into ellipses while maintaining the orientation of the threshold one, but the shapes were not completely same, especially, at Blue center. The comparisons between visual color differences and their colorimetric counterparts, in CIELAB ΔE units or threshold units, show linear relations at both color centers, but the slopes were, in general, not equal to 45o and differed for all directions. Thus the suprathreshold color differences can be derived by enlarging the thresholds linearly but not uniformly, i.e. with different ratios for individual directions.