Abstract
After Hering, colour is described in terms of six unique hues (white, black, red, green, yellow, and blue). Yet, there is no objective psychophysical procedure for establishing the nomenclature of unique hues. It is still unclear whether, say, purple is unique or not. A new method has been developed to ascertain the set of unique hues. The procedure was as follows. Trichromatic human observers were presented with pairs from a set of 21 Munsell chips with an instruction to evaluate whether they share any hue (partial colour match). Sets of chips that have identical partial colour match (matching classes), and largest sets of chips all of which partially match each other (chromaticity classes), were derived from the matrix of responses.
A chromaticity class is proved to consist of all chips which contain a particular hue (referred to as component hue). Thus, the number of chromaticity classes shows how many component hues an observer employed in their decision making. A matching class is proved to contain all the chips with identical component hues. We found four matching classes which contained only one component hue. Moreover, these classes consisted of just one chip. We concluded that these four Munsell chips (5Y, 10B, 5R, 10G) represented four unique hues.
While our results are in line with the previous studies of unique hues, the partial colour matching technique has an advantage that it is based on the data of type A, in terms of Brindley's classification.