Perkup has some great ideas which I agree with. Yes, I remember the toothpaste commercial. It's a nice flash back.
Some drugs can yellow hair as can hard water and cigarette smoke.
My hair is naturally silver, not white and I have never had yellowing. My sister has Snow White hair with no yellowing problems either.
For most people, it takes a while for the back, usually around the nape area to lighten up. That area will remain naturally dark or salt and pepper for a while. Sooner or later it too, will change. I would just hang in there and just use a hair brightens to try to keep the yellow color at bay.
Almost forgot...Make sure you do not use a curling iron on your hair if it has hair spray on it. It will burn and make your hair yellow too. Keep the iron at a moderate heat and go easy on styling products.... Like butter, the product can burn and discolor.
Best of luck.