It seems like you are having staining of teeth. It occurs due to many causes and on consumption of staining substances like tea, coffee , iron containing medicines etc and some people are more prone to it. You only need to get your scaling and polishing done at least once a year

In most of the cases, a simple scaling/cleaning can eliminate the issue. Once in six months get a scaling /cleaning done . Possible causes of these black marks may be due to Eating foods that are loaded with sugar or drinking carbonated beverages. Consuming too much tea or coffee can also cause staining. Not rinsing your mouth well or not brushing your teeth properly can also cause black spots. Some antibiotics tetracycline and doxycycline are known to discolor teeth when given to children whose teeth are still developing (before age 8). Mouth rinses and washes containing chlorhexidine and cetylpyridinium chloride can also stain teeth.