Volatile sulfur cpds. Usually the yellow color is a volitile sulfur compound. These compounds form when bacteria in your mouth metabolize protein present in the food you eat & old cells that are shed from your cheeks, tongue etc. Remove food & debris by thorough flossing, brushing, & scraping the surface of your tongue. Also brush & swish with chlorine dioxide (ie closys) to kill these gram (-) aneurobic bacteria.
Yellow tongue. You may be breathing with your mouth open.