Thank you for writing Justanswer.
This is not as uncommon as you might think. In fact I just had another question about a yellow tongue two days ago.
It is common for the tongue to become discolored due to food, smoking and bacteria.
Does it go away at all when you brush your tongue, or is it that it always comes back?
Your problem is obviously not a yeast infection, because the yogurt, as well as the anti-fungal pill, which likely was what he gave you, would have cleared it up.
This article from the Mayo Clinic discusses "yellow tongue" in more depth:
They recommmend using a solution of hydrogen peroxide as a mouth rinse, as well as quitting smoking, if you smoke.
If you have questions or comments, please reply so that I or another expert can address them.