Darkening of the colour of the teeth can be the result of many other things. Drugs, smoking, food colourings, environmental contaminants, bad hygiene, diseases and health conditions etc can all be the reason. Unless you are using really low quality products such as RTD or teabag green tea, and in unusually large dosage, prolonged usage of green tea does not result in darkening of the teeth.
If what you mean is tea stain, green and white teas are actually the least possible of all the tea categories to be associated with such problems. Normal oral hygiene habits take care of it much easier than if it is coffee, soft drinks, or any other sugared beverages.
