It could be from drinking coffee. Use a tongue scrapper and use every time you brush. Some toothbrushes come with a tongue scrapper, but they don't work well. Just be careful when using the tongue scrapper because you can gag easily. Good luck!!!
Yellow color may come from extrinsic stain, such as, food, drink, or chromogenic bacteria. Intrinsic yellow color may come from consuming too much carrot or food containg beta-carotene, or jaundice.