When I wake up, my tongue is yellow. How do I fix that?

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.
Use tongue cleaner. It is from toxic builtup overnight and bacterial growth. Get "u" shape tongue cleaner and clean the tongue daily after brushing the teeth. Have a regular daily bowel movement if not. Eat high fiber diet with lots of fruits and vegetables.