Choosing multiple color versions from within the same theme is possibe, i did it in Simplicity. But not all theme authors think its needed I guess.
Doing it as a mod, will be difficult, because it won't know how color versions is present, and some themes change more than just the style.css. Helios change just the graphics, and NOT the style.css...
While Themis has 3 color versions built-in, admittedly only for the admin, but it can be adjusted slightly.
Thats why I suggested just setting up more themes with the different colrs , and just plain and simple provide links to them.