A yellow tongue can be caused by a build-up of bacteria and debris on the surface of the tongue. This can happen when you don't brush your tongue regularly or if you have a medical condition that causes excess saliva production. A yellow tongue can also be a sign of jaundice, which is a condition that occurs when there is too much bilirubin in the blood. If you are concerned about your yellow tongue, make an appointment with your doctor to get it checked out.