I don't want to scare you and I'm probably wrong but, usually yellowing of parts of the body can be caused by a liver problem, like with the skin when it becomes jaundiced. That's all I can think of, but it's probably completely innocent and could just be linked to your diet. If you feel fine otherwise then it should be okay, but if you're concerned ask your dentist perhaps. Are you a smoker? As that could cause some discolouration of the tongue due to the nicotine. Hope that helps and all the best.