In the early 20th century Albert Munsell did a series of empirical studies to determine exactly how people see colors relative to one another. The results are expected, but fascinating.
(Also, I really need to expand the Munsell Wikipedia article, but Albert Munsell was dead before much of the empirical work that went into the 1929 Munsell Book of Color – by that point his son was running the company – or into the 1943 Munsell Renotations, which were based on the work of the Optical Society of America.)
http://en.wikipedia.org/wiki/Munsell_color_system