This is Non-medical belief not testified any where scientifically.
when any person vomits, the contents of the stomach comes out. and they are mixed with fluids of body. That makes the colour of contents yellowish, or slightly greenish. Yet mostly color depends what one has eaten.
and secondly, the teeth becoming yellow is due to long process of decay, deficiency of calcium, frequent chewing of various items. and mostly Tea addicts develop yellow teeth, and also those who smoke heavily also are victims of yellow teeth.
To get sparkling teeth, one need to improve diet, Eat lot of fruits, raw salads, chew the food well, avoid all addictions and intoxicants. exercise well, and have good rest at night and also have rest in day time for few minutes. As women have to do all domestic work , the load on them is more. So, napping or 'waam-kukshi' a short nap after lunch is called waamkukhsi is recommended by Ayurveda. and this enhances the age also.