Theaflavins and thearubigins, the dark tooth staining molecules that love to stick to teeth, also have a very high affinity for milk proteins called caseins that are found in animal milks. Caseins surround the staining molecules and keep them from sticking to your teeth. This binding between milk proteins and theaflavins/thearubigins is enhanced by milk fats, so the higher the fat content in the milk you add the greater the stain reduction.
Unfortunately, this effect on staining doesn’t apply to soy, rice, almond, or any other type of non-animal milk (using non-animal milks may actually cause more staining), but it will work a bit with creamers because they contain small amounts of caseins. The bottom line is choose whatever tastes good to you, but know that cow milk or any other animal milk with a high fat content will give the greatest reduction in staining. Sorry skim milk fans.
Many people think that because milk binds theaflavins and other polyphenols, which have many health benefits, adding milk to your tea will mean that your cup of tea will be less healthy. However, studies conducted that actually looked at absorption and plasma concentrations of the healthy compounds in tea saw no negative effects from milk. Once the tea makes its way down into your digestive tract, your body does a great job of separating the milk from the healthy tea compounds so that you will absorb them.
If you are able to drink milk from animals and like the taste of it in your tea, this is a pretty neat trick for keeping your teeth white.