Hi.. Thanks for writing on HCM.. Black discoloration of tongue after dental treatment that is now yellow can be due to a condition known as Black Hairy Tongue and it can occur due to a number of conditions like overgrowth of bacteria and fungus over the tongue, entrapment of dead cells over the tongue papillae and when the Papillae do not shed as per its normal cycle and there is elongation of the papillae and more of debris and dead cell entrapment as well as more growth of bacteria and fungus. Tobacco chewing, smoking and excess intake of tea and coffee can also be contributing factor. You should consult an Oral Physician and get evaluated and in case of fungal infection along with maintaining good Oral hygiene you will have to take anti fungal medication like Nystatin mouthwash and clotrimazole lozenges.. Taking more of Probiotics in diet like Yogurt and Garlic will help.. Use a tongue scraper to clean your tongue.. Gargle with 1:1 solution of hydrogen peroxide and water as well as you can dip your toothbrush in this solution and brush your tongue gently.. Take more of hard foods like apple, carrots and other things as they will clean the tongue by friction.. Drink plenty of water.. Limit intake of tea and coffee.. Hope this information helps. Thanks and regards. Dr.Honey Nandwani Arora.