Yellow on the back of my tongue?

Hard to diagnose: Without seeing it firsthand, it is kind of hard to diagnose. Possibilities may be oral thrush which is caused by a fungus. Another possibility may be that the tongue is not routinely brushed when you brush your teeth and thus would leave a film behind. Obviously, the best thing to do is to go see your dentist, so they can make an accurate diagnosis.

Probably build up: Discoloration of the tongue is usually just that--discoloration. It is not usually associated with a disease. Yellow discoloration is typically build up that has collected on the rough surface of the tongue and can be removed over time by daily use of a tongue scraper, which is readily available in the pharmacy section of most stores, or can be obtained at a local dentist's office.