Sixty-five wool sample pairs around eight colons centers were used to investigate the change of colour differences caused by four viewing parameters: lightness of background, sample separation, luminance, and magnitude of colour difference. The average size of colour difference was 13 CIELAB Delta E units. Each pair was assessed 20 times by a panel of 13 normal colour-vision observers using the gray scale psychophysical method. The viewing parameters investigated did not greatly affect the perceived colour difference. The largest discrepancy was found between the gray and white background conditions, i.e., the same pair of samples shows a 30% larger colour difference against a white background than against a gray background. The present results were also used to compare,vith those of the authors＇ earlier study(2) using small colour differences. It showed that there is a large difference in characteristics between the large and small colour differences. The visual data were also used To test five colour-difference formulae: CIELAB, CMC, BFD, CIE94, and GLAB. The results showed that the GLAB formula, developed by the authors, performed marginally better than the other formulae.