Consistent grey-level ordering for iso-luminant and iso-saturated colours

If a red and a blue patch, of equal brightness and saturation are presented to participants, would they all assign a darker grey to the blue patch than to the red patch? In our experiments we selected 8 coloured patches in cone-opponent space that lay on a circle in the 20 cd m-2 plane, displayed on a calibrated CRT. Using heterochromatic flicker we established for each colour and each of our four participants their corresponding iso-luminant point and by adjustment their iso-saturation setting. Participants were presented (3 times) with all possible 56 pairs of colours and asked to set a grey level for each item in the pair. From the participants' grey settings we were able to recover their individual distance matrix for the 8 colours used. Three observers showed a high degree of inter- and intra-observer repeatability. The resulting distance matrices were roughly symmetrical (distance between AB the same as for BA). Applying multidimensional scaling to the distance matrix showed that the distances were well represented in a 1-D scale.