The back of my teeth are dark yellow. what does this indicate?

May be normal.: Back teeth are darker than front teeth. Top teeth are lighter than bottom teeth. However, teeth can stain from pigment containing foods. See your dentist to make sure everything is ok. You might benefit from cleaning or bleaching.

Need cleaning: They could be coated with stain from something you eat or drink. Common culprits are coffee and tea. They could also be coated with tarter or calculus. These must be removed by a dental professional. Possibly your teeth are worn on the tongue side due to acids. This is common in bulimia. You should see a dentist for a better diagnosis.