Why are the back of my teeth so stained?

I have been drinking black coffee from a straw to not stain my teeth. The front of my teeth look nice, but just now I saw the back of my teeth & they are severely stained. I do eat berries, but I brush right away or drink water usually right after. My God, what can I do about this? Although the back of my teeth... show more I have been drinking black coffee from a straw to not stain my teeth. The front of my teeth look nice, but just now I saw the back of my teeth & they are severely stained. I do eat berries, but I brush right away or drink water usually right after. My God, what can I do about this? Although the back of my teeth can't be seen this does concern me & I don't like it.
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