An experimental approach is described for measuring colour discrimination thresholds of human observers. Special software was developed,for the accurate display of colour pairs on a high resolution CRT, using serial feedback from a spectroradiometer. Discrimination thresholds between a test and a target colour are determined by repeatedly showing an observer a circle composed of four separate quadrants, one of which has a different colour from the other three. Three quadrants are of the test colour and one of the target colour, or vice versa. Observers are asked to select the quadrant that differs from the others. An affect-experiment is described where hue-dependent effect affecting hue discrimination are investigated. Eighteen hue threshold values around the hue circle, at constant L-10* = 51 C-ab,C-10* = 25, were measured for three observers. Hue thresholds were found to vary around the hue circle, exhibiting an abrupt change in the blue to purple region (240 degrees <= h(ab,10) = 300 degrees) This change is not fully accounted for by any CIELAB-based colour difference formula, including the most recent CIEDE2000 formula. (c) 2005 Wiley Periodicals, Inc.