Geez. ok , so she is pretty hot except her teeth are slightly discolred? not butter yelow, just a litle dingy? SO SIMPLE
Go ahead and establish a relationship if you can. Then on a night when the two of you are just sitting there talking bring it up.
You: Hey, I got something I gotta tell ya!
Her: what?
You: hmmm well, the other day i noticed something wrong with your teeth.
her? What? is one chipped? do I have a cavity? etc. etc.
You: they just seem to be a little yellow, you been eating something to make them discolored?
Her: not that I can think of!
You: I had a friend once with that same problem...they make this whitening gel, it works preety good, next time we go by a store i'll point it out to you.
Her: Yes please do! I never realized!
It don't matter if she knew they were yellow or not. Now she wants to change it! And, at the same time you have done her a favor without being to critical. Would you let a friend stand there with a booger in his nose? Hell no! He'd be embarassed at first that you told him, but thankful for giving him a chance to fix it later. So will she! Adopt this to your own personality. Good luck!