there are many reasons why color might change, tea is not that
staining (although the "black teas", lipton type, really are). as we age our enamel gets thinner, revealing by transparency the color of dentine underneath. and then it depends on how porous (or little porous) and how thick the enamel initially was.
In other words don't worry, one day we will all be old and toothless
anyway, until then may I suggest
-a very smooth brush + good technique
-white clay reduced in ultrathin particles
-calcium carbonate : very moderately abrasive, can be mixed. maybe not everyday.
- montmorillonite clay paste used as a teethmask (apply it, leave it for 5mn then spit it as elegantly as you can and rinse)
baking soda is significantly abrasive
bleaching is the worst idea in the world : makes dentist rich but also makes enamel more fragile.
anything acid is not a good idea.
I like weleda toothpastes, work well and good composition. the salted one yet I am not keen on; vegetal (green packaging) is my favorite.
I rinse with warm spring water. sometimes add an hydrosol, but not if it may interfere with tea flavors. emusion of warm spring water and olive oil is also a good rinse.