Are there four, six, or seven unique hues? by Christopher W. Tyler

Supplementary Material
The colors visible in the color circle are shown in the upper part of Fig. 1. The left-hand image shows the standard color circle, in which yellow, magenta and cyan colors appear as narrow ridges between large spans of red, green, and blue. When the whole image is pushed through a compressive nonlinearity (right-hand image), the color ratios change dramatically, with the yellow, magenta, and cyan zones expanding to become at least as wide as the red, green, and blue ones. This reconfiguration supports the concept of six fundamental, or unique, hues, consistent with the prediction of the complementary color system based on the three cone classes and their complements (see Pridmore, 2013, for a review of the neurophysiological support for this analysis).

Fig 1. Two versions of the color circle, with test disks for viewing their afterimages. View as close as possible for optimal effect

Fig. 1. Two versions of the color circle, with test disks for viewing their afterimages. View as close as possible for optimal effect

To view the afterimage colors, one black dot should be fixated for 10-20 seconds, and fixation then transferred to the center disk. It will be seen that the afterimage that appears emphasizes the cyan, yellow and magenta colors across the circle diameter (as shown by the data in Fig. 2), even for the Standard linear color circle but more strongly for the Compressive one. This demo validates that the complementary pairings are not red vs green, but red vs cyan (aqua), and green vs magenta (purple), consistent with the concept of 6 fundamental colors around the color circle.
Data for one subject with strong afterimages

Fig. 2. Data for one subject with strong afterimages, showing that tight adherence to the complementary direction in CIE space, passing through neutral gray (not the yellow neutral point of red-green opponency). Black circles: loci of the adapting stimuli. White circles: afterimage loci, connected to the respective adapting loci with white lines. Triangle: monitor color gamut.

 

Reference
Pridmore, RW (2013) Single cell spectrally opposed responses: opponent colours or complementary colours? J Optics 42: 8–18.