Yellow Tongue. A yellow coating on the tongue is also caused by inflammation of the papillae, which then collect yellow bacteria. Smoking, mouth breathing, dehydration and fever all cause the papillae to become inflamed.ellow Tongue. A yellow coating on the tongue is also caused by inflammation of the papillae, which then collect yellow bacteria. Smoking, mouth breathing, dehydration and fever all cause the papillae to become inflamed.