I should not really call this a toy but I like playing with such things. It is actually a comprehensive introduction to Colour Theory hosted at Brown University. Information on both electronic and pigment-based colour theory is covered and contains exercises that allow the user to discover for themselves the theories. Colour mixing models and colour contrast theories are also covered as well as a bibliography of books on colour theory. The technology behind the site is driven by Java applets so the user can explore various aspects colour theory by interacting with the site. Try it out you will see what I mean.
A big thanks goes to Linn of the Embroideress who drew my attention to it.