Tension ring color generally matches that of the dial, though examples exist of black dials with white tension rings, and white dials with black tension rings. In addition, we also see silver (reflective) tension rings as well.
The tension ring may or may not have radium luminous plots at each hour. If there are plots, they are more likely to be found on a white/silver tension ring.
What to watch for: when a 5907 features a tension ring with luminous plots, but the hands are non-luminous. Maybe it is original, but makes no sense to me!
My understanding is that the crystal diameter is 30.9mm, though I have not personally measured, and the tension ring is smaller, though I do not know the diameter.