At 28 years, your hair is prematurely graying  and that means that the oxidative damages in your body have increased. Please understand that hair turning white doesn't mean your hair are unhealthy...or they are turning white just because of shampooing etc...it means your body has started to age (prematurely)and is unhealthy. I sincerely suggest that you Visit the dermatologist allopathic near you. Turning white hair to black is difficult but you can prevent future greying by MODIFYING LIFESTYLE . Sleep by 11pm, wake up by 6 am and eat a HEALTHY AND BALANCED DIET. Exercise 45 min everyday. This helps the metabolic processes in the body work at par. Healthy body = Healthy hair