The performances of colour-difference formulae were investigated for predicting very large colour difference based on visual evaluation of object surface colours. The experiment of psychophysical visual assessment of 60 sample pairs with ratio judgement was carried out on 18 observers. The evaluation data of visual colour difference were btained. The data were used to evaluate the two common colour-difference formulae, CIELAB and CIEDE2000, and the two latest CAM-based colour-difference formulae, CAM02-UCS and CAM02-LCD. The detailed comparison and analysis indicated that CAM02-UCS and CAM02-LCD outperformed others for very large colour difference. In addition, the experimental data were employed to test the power law model proposed by Attridge et al. and the consistent result was obtained.