Topaz and Mystic do have noticeable differences.
Agreed, in bright light, both look identical. But, the topaz colour does look a clean colour with less aluminium in the paint. Therefore, less metallic effect.
Mystic has more depth to the paint and appears to have a tint of purple to it. The colour lifts with light at certain angles.
Almost identical light conditions and taken at 1280x1024 originally: