Just to pick up on a couple of points from earlier - I had always pictured the Silmarils as being white light, perhaps with a yellow tinge, a mix of the light of the Trees. I'd certainly always thought of them as being all the same colour, although the idea of Morgoth sitting deep in the bowels of Angband wearing a traffic light on his head is quite appealing.
And for the Rings - my feeling is that the Elves could, potentially, have made their own without advice from Sauron. They could certainly make "magic" things by themselves, and they had great knowledge both from their own study of Arda and from (long ago) the teachings of Aulė. The Three were instruments of stasis and preservation, not command and control, but a Ring-shape - circular, enclosing - is appropriate to both tasks.