Hairs further turning to grey can be prevented by a procedure called as shiro virechana.

The natural colour of hair is due to the cells in the hair follicles known as melanocytes which generate a pigment which gives colour to the hair. When melanocytes stop producing these pigments, due to increasing age,decreased nourishment,protein deficiency etc your hair starts to lose its natural colour which leads to graying of hair...try to have healthy diet at regular intervals,drink more water,manage stressful situations smoothly if possible avoid them,biotin supplements helps you..Consult dermatologist they will do the needful.