An experiment was carried out to study large colour differences. Two-hundred-and-ninety-two wool pairs were prepared with an average difference of 12 CIELAB Delta E units. Visual assessments were conducted using gray scale and pair comparison psychophysical methods by a panel of 20 observers. The results from the two psychophysical methods were used to test colour-difference formulae and compare their differences. The gray scale results were also used to compare with the other large colour-difference data sets, i.e., BFB, OSA, and WW. The present results together with part of the other data sets were combined to form a single data set. A modified CIELAB colour-difference formula was developed having a structure similar to that of CIE94. It is named the GLAB colour-difference formula.