A new visual threshold lying between the colors that appear to contain gray and those that appear fluorescent has been determined as a function of dominant wavelength at high purity on a white (7000 K) background. The function is found to be an approximately constant multiple of the previously known purity threshold; it is independent of purity and hence is named chromatic threshold. The value of the function for any given chromaticity is shown to be a measure of its chromatic strength in (a) neutralizing its complementary color, (b) producing Munsell Chroma. It is considered also in terms of the Hering—Johansson “Natural Color System,” where it appears to specify the “clear” series, and in terms of the Ostwald System.
RALPH M. EVANS and BONNIE K. SWENHOLT, "Chromatic Strength of Colors: Dominant Wavelength and Purity," J. Opt. Soc. Am. 57, 1319-1324 (1967)