If you just drop the HSL "saturation" to get a gray, you quite dramatically change the lightness in many cases.
More intuitive models:
Munsell Color System (related to rigorous experiment in the "Renotations" of the 40s. Many later models were attempts to model Munsell space with a closed formula instead of the lookup table that defines the Munsell System itself): http://en.wikipedia.org/wiki/Munsell_color_system
CIELAB, the de facto standard for many decades in colorimetry, used for example in many color management modules, defined in the mid-70s. (pretty weak article) http://en.wikipedia.org/wiki/Lab_color_space
More intuitive models:
Munsell Color System (related to rigorous experiment in the "Renotations" of the 40s. Many later models were attempts to model Munsell space with a closed formula instead of the lookup table that defines the Munsell System itself): http://en.wikipedia.org/wiki/Munsell_color_system
CIELAB, the de facto standard for many decades in colorimetry, used for example in many color management modules, defined in the mid-70s. (pretty weak article) http://en.wikipedia.org/wiki/Lab_color_space
IPT: I have no great links handy, but there's a mention here http://documentation.apple.com/en/color/usermanual/index.htm...
CIECAM02, the 2002 recommendation of the technical committee scientists studying this stuff, http://en.wikipedia.org/wiki/CIECAM02