I used to have an abundance of "yellow" on the back of my tongue like you. About 5 years ago, I incorporated a gentle (albeit plastic) tongue scraper into my daily routine (alongside brushing and flossing). The yellow stopped coming back after about a week of use.
My assumption what that it was simple, normal bacteria growth (or its waste products) in the mouth. I haven't eaten an abundance of carbs (sugar) for years, so I haven't seen a cavity in about as long, so maybe it was a pH related occurence.
Regardless, the "yellow" was unsightly -- and the tongue scraper worked.
A tongue scraper helped me but, as said by justin, that's reactive.
On ancestral eating, the yellow went away but reappears occasionally. If I eat a meal that causes a temporary flare-up of GERD I can expect a yellow tongue for a while. I figure it's a combination of reflux and bacterial overgrowth.
Eating too late in the day definitely is a trigger particularly if the meal included carbs. Interestingly enough, beef never causes a problem even if I eat heavily and neither do salads--the foods I have to watch are fruits and starches late in the day.
Educate yourself about fasting. I've heard those are toxins coming out on your tongue, and that you should fast until your tongue is nice and pink. It really does work....usually takes 10 days for mine to go pink. Then you can use a tongue scraper as part of your morning dental hygiene routine to help keep it clear.
I'm guessing oral thrush (Candidiasis), thus the Nystatin. You need to take that for a week or so for it to kick in though (doesn't go away in a day). Did you take the full course? Check with your doctor for other options since the Nystatin didn't work. Other than that, look up oral thrush on the net and see your options (I don't recall ever getting it).
My guesses. The yeast overgrowth is due to changes in the mouth/diet/immune system. So make sure you're taking your vitamin D, look into some oral probiotics (to help crowd out the thrush), maybe tea tree oil gargles, and see if there's specific things in your diet that seem to aggravate it.
Oh, and make sure you sterilize/replace your toothbrushes so you don't keep recontaminating yourself with that particular strain.
I used to had that problem. But eventually it went away on its own. Though i changed my diet a lot. I eat very limited types of food and mostly follow ray peat style diet. I think yellow tongue got to do with liver problem. Good protein helps with liver function. I am not sure, but i think i saw improvements in tongue color and over all health after i stopped eating all kind of polyunsaturated oils. I hope it helps.