I don't want to give you any false hopes but diet alone will not help in preventing premature greying of hair. It could be due to increased stress, smoking which produces free radicals in the body which are responsible for accelerating the ageing process leading to premature greying and wrinkled skin. You must try to quit smoking first and adopt a healthy lifestyle by eating right and staying active. Use a herbal shampoo and oil which is rich in amla (Indian gooseberry) as it helps to prevent premature greying. You can prepare it at home also. Take some amla pieces and dry it in the sun. You can also use amla powder. Mix it with some coconut oil and boil the mixture till it turns black. Cool down the mixture and store it in a bottle. Regular application of this oil helps to arrest premature greying. Hope these suggestions will be helpful.