The difference that I would see is that most designers probably don't want to be restricted to just the colors that are defined in the spec and it would be useful to have the colors associated with their purpose (e.g. text color, background color) so that they could be changed without having to change them throughout the various rules.
Giving names to colours in a universal spec like HTML5 would be asking for trouble. You'd be on a never-ending treadmill of new colours. Constants (however that's done, e.g. SASS variables) is clearly the way to go.
