Can yellow teeth be permanent?

No, they are not 'permanent'. It's probably because you aren't brushing your teeth regularly, smoking, or eating some type of food or consuming a beverage that caused your teeth to become yellow. You can make them white again by scheduling an appointment with your local dentist, and asking them to clean your teeth up, be sure to ask the dentist that you want your teeth to be white so he can provide the necessary tools to make them white again.

If you want to do it for free, although it will take a while and perhaps be a little complicated, read the link I provided in the comments (Yahoo is broken, if I post it here, you wouldn't be able to see my answer because they'll "ghost" it.) and follow the instructions.

Brush twice a day, floss at least once a week (no one flosses everyday), and rinse your mouth with a whitening mouth wash after you brush. If they still remain yellow, it may be due to something other than your hygiene and you should ask your dentists about it at your next appointment.

Unless you have a medical condition, yellow teeth can be reversed. These days, people go to the dentist for whitening treatments.