The visual data obtained from the color discrimination threshold experiment carried out in the iso-lightness a*-b* plane of CIELAB color space were used to test the four classical color-difference formulae, CIELAB, CMC, CIE94 and CIEDE2000, in terms of performance factor PF/3. For the original forms (kC=kH=1), CIEDE2000 outperformed others, followed by CIE94 and CMC, with CIELAB the worst.For the color-difference formulae with kC and kH optimized, CIEDE2000 still performed the best, while the color-difference prediction performances of other three formulae were almost the same; except for CIELAB improved obviously, others were enhanced a little in comparison with their individual original forms. The CIEDE2000 formula performed excellently in the yellow and blue color regions, but its color-difference prediction performances in the gray, red and green regions remained to be further improved.