Alternative to adding milk to tea and switching to less oxidized teas, you can try brewing less traditional cups of “tea” to decrease tooth staining. Two good options are rooibos tea and yerba mate. Both aren’t traditionally considered tea and will stain your teeth much less.
Rooibos tea is an interesting type of herbal tea made from the plant Aspalathus linearis which is grown mainly in South Africa, and is naturally decaffeinated. It is not technically a tea and actually a robust tasting herbal tisane (herbal tea). If you are looking for some Rooibos tea at the store it sometimes is called red bush or red tea, although black tea also goes by this name as well.
Rooibos has been referred to as a coffee drinkers tea because of its nutty flavor qualities, and it has a much different anitoxidant/polyphenol profile than typical teas because it is a tisane and not a tea. In fact, Rooibos leaves do not contain any catechins (a type of polyphenol) so none of the super staining theaflavins or thearubigins can be produced in the tea leaves during processing. This means that Rooibos tea will be very low staining and an all around a nice decaffeinated herbal tea to drink.
If you haven’t visited South America, you may have never even heard of yerba mate; it is extremely popular in Uruguay. Yerba mate is made from the naturally caffeinated and nourishing leaves of the South American rainforest holly tree (Ilex paraguariensis), and it is said to have the strength of coffee, the health benefits of tea, and the euphoria of chocolate. Mate is typically enjoyed out of a special mate gourd using a metal straw.
Since yerba mate comes from a type of holly tree it does not contain theaflavins and is processed much differently than traditional teas. Therefore, it has much less staining potential (probably about the same as green tea). It also has a fairly high caffeine content, somewhere between that of green tea and coffee, so it can be a good low staining alternative to black tea.