The Dimensions of Colour - by David Briggs (Website).

"All painters, whether working in traditional or digital media, are in a real sense navigators in space. Whether they are aware of it or not, each touch of colour they apply can be considered, using various systems, as a point within a space defined by three dimensions." - an excerpt from David Briggs's website "The Dimensions of Colour".  

His site hosts a large amount of well organized/concise information coupled with informative illustrations, including 3D modeling and animations, all of which make this one of the best online color theory resources that I've been able to find.  Thanks Mr. Briggs!  

Direct link:  The Dimensions of Colour