Hair graying is natural ageing, programmed in your genetic makeup. some start earlier some later. Knowing your age would have helped a little.

A remedy using black sesame seeds is mentioned at

http://health.groups.yahoo.com/group/ayurvedaonline/message/8670

Bhringraj juice is strongest remedy known to ayurveda. Frsh juice of bhringraj leaves is used in India to return hair to natural shade in 3 months. Now getting juice is impossible, except in Bhringraj herb areas and that too only in July-August. An oil made from Bhringaraj, known as Mahabhringraj oil is available on the net. You can use this oil and see if it can help. In the absense of your age, it may give other benifits, even if shade of hair does not change.