Hi sir...
yellowish discolouration may be due to soft and hard deposits(plaque and calculus) ,dental flurosis, soft drink sains...
You need to go for scaling and polishing( cleaning) followed by Tooth bleaching(teeth whitening) procedure..
Next Steps
kindly visit dentists for furthur consultation and treatment....
Health Tips
Do scaling forevery 6 months..
Do brushing and flossing twice daily...
Do rinse with mouth wash twice daily after food.