Why do different pure hues have differing lightness values?
The perception of lightness is complicated and depends on many variables. However a simple way to look at this question, that does explain most of what is observed is to think about our relative sensitivity to light of various wavelengths through the visible spectrum. In general we are more sensitive to wavelengths in the middle of the spectrum (approximately green) than to those at the ends (approximately blue and red). A pure green hue will be made up of light that we are very sensitive to and thus generally have a higher lightness than a pure red, blue, or purple (which is made up of red and blue). Of course it is possible to create stimuli that do not follow this trend, but for reflecting objects, or lights with equal amounts of energy, this general rule will hold. Usually highly saturated greens and yellows have a relatively high lightness, while reds, blues, and especially purples have a relatively low lightness.
