What color does yellow teeth turn into? Is it possible to turn it back?

Whitening. Whitening your teeth may reduce the yellowing that can occur over time. See your dentist for evaluation and their at home whitening program details. Teeth should be cleaned @ office before starting a whitening program. Good luck.
See your dentist. Your dentist can bleach your teeth. There are the take home trays and the in office power bleaching option. Discuss this more with your dentist. If the bleaching of the teeth cannot get the teeth to the white color that you desire then veneers would be the next option.