Threshold-discrimination ellipses were obtained by three normal observers, at five color centers, by the use of two experimental devices, which provided light-surface colors, and the same method and experimental conditions. The ellipses obtained for each center in both devices were quite similar when the sample distributions were appropriate and slightly smaller for object color than for aperture color. The ellipses predicted by three recent color-difference formulas based on the CIE uniform color space (CIELAB) were compared with those experimentally obtained by us as well as by other researchers who analyzed a greater number of color centers. The color-difference formula proposed by the CIE Technical Committee 1-29 [Color Res. Appl. 18, 137 (1993)] provides the best prediction of the semiaxis relationship for all the experimental datasets used, confirming their good performance in previous works [Appl. Opt. 33, 8069 (1993)], although the differences with respect to the experimental results are higher than those attributable to the interobserver variability.