There were a few comics in the geoff johns series where Hal Jordan had a blue ring as well as a green. If I recall correctly the two kind of fought for his mind. The colors don't really mix when a user has multiple rings to make turquoise etc. Usually one color dominates at a time. White was not really a mix of each of the colors exactly, but the essence of the first life in the universe, from which the other color-emotions split. And the guardians only mess with green, they cannot control other colors (though two split off to make the blue Corp). The yellow impurity, the primal creature parallax, was removed a long time ago from the central battery. Young lanterns still have issues with yellow until they gain experience because fear naturally interferes with will.