An investigation of the correlation between visual colour assessment and instrumental colour
acceptance determination using regression analysis has been carried out. Three colour-difference
equations, CIELAB, CMC(2:1) and CIE94(2:1:1), were studied in order to determine which is the best
for generating a uniform colour space/microspace for allocating the colour population in shade
sorting. Determination of optimum colour tolerance for further shade sorting was also undertaken.
Some 1320 pairs of dyed samples distributing around 20 shade standards were measured
instrumentally and also evaluated visually by a panel of 32 observers. Percentage rejection was
plotted against colour difference and different mathematical regression relationships were then
imposed. As a result, both CMC and CIE94 showed better correlation between the two colour
assessment methods than the CIELAB colour-difference equation. Consequently, optimum colour
tolerance limits were determined for subsequent development of shade sorting, with the findings
being equally applicable to colour acceptance (shade passing).