The performances of modern colour-difference formulae were investigated to evaluate very small colour difference at human eye’s threshold level based on object surface colours. A panel of 21 observers performed the visual assessment to 100 sample pairs using perceptibility method. The validity of the visual results was verified via the observer variation estimation and the comparison with the BFD's and Witt's published data. The detailed analysis indicates that CIEDE2000 recommended by CIE performs best and CIELAB the worst among the six modern colour-difference formulae, namely CIELAB, CMC, BFD, CIE94, LCD and CIEDE2000, for predicting very small colour difference at the level of human eye's threshold.