If there is one downside to red wine, it's stained teeth -- that purple-grey coating that is a dead giveaway you've had one too many. Other dark beverages, like coffee and grape juice can stain teeth too, but none do it quite as dramatically as wine. So what gives? "There are three things that influence the staining of teeth," says Dr. Mark Wolff, a professor at NYU's College of Dentistry. "The first is the intensity of color, or chromogen. In red wine, that red is very intense."