Definition for Unique hues
From Biology Forums Dictionary
Unique hues could also be called psychologically pure. For example, a unique green has no yellow or blue content. A unique yellow has no green or red content. Four unique hues have been identified: red, green, yellow and blue. There are monochromatic wavelengths that produce unique green, yellow and blue. Unique red, however, can only be produced by mixing a little blue light with even the longest wavelengths. A wavelength of 700 nm for instance contains a small yellow content. When blue light is mixed with 700 nm the yellow is cancelled and a unique red is produced that contains no yellowishness or blueness.