it's very hard to tell because of the now questionable quality of pre-restoration OS episodes. everybody sees a different shade of grey or white. even book scources disagree, and, according to a pic I saw on an earlier post, even the Shooting Model of the enterprise is variably colored.
once scource i read states that the color is "Thermal Grey"
i had to use gradients due to the limitations of my art program, though I did count the number of rays on the original to get it right. the rings, however, were zero black and Grey 64 in overlay mode.