Drinking red wine, some people’s teeth, gums or lips will get a very distinct purple stain, while this does not happen at all for other people.
I have a few guesses as to why:
The smoothness of the outer layer of the teeth varies from person to person, and that leads to more “shadow” or surface for purple color.
It happens to everybody, but some skin tones provide a sharper contrast to the color.
It depends on how long the wine touches the lips and teeth: it would happen to everybody if the same amount of wine stayed in the mouth the same amount of time.
It actually happens to everybody, but it becomes more visible as some people open their mouths wider when speaking.
The questions are: Why and on which part of the mouth (lips, gums, teeth) do the stains appear? Why does it happen more for some than for others?
This question has been asked elsewhere (e. g. on Yahoo Answers) without satisfying answers.