Unique hues and principal hues

This note examines the different concepts of encoding hue perception based on four unique hues (like NCS) or five principal hues (like Munsell). Various sources of psychophysical and neurophysiological information on hue perception are reviewed in this context and the essential conclusion that is reached suggests there are two types of hue perceptions being quantified. Hue discrimination is best quantified on scales based on five, equally spaced, principal hues while hue appearance is best quantified using a system based on four unique hues as cardinal axes. Much more remains to be learned.