Ivare

Well, I'm not exactly sure. Since they held the light of the Two Trees, normally they must have glowed a bit with silvery light with just a hint of goldenness, but when they encountered other light reflected it back in whatever color that light was, only brighter, more radient. I think that if you were up close to them the glow wouln't be overpowering, but the sort of thing that you notice even if you're far away. They had to be small, since at times Feanor wore them apon his brow, unless he had a really big head. (wait a sec...) Tolkien thought at least at one point that they were also spherical in shape, as is mentioned in Lays of Beleriand in the unfinished poem "The Flight of the Noldoli"
But now we come to another question- how didFeanor wear them?