Research has found that grey hair is primarily caused by a buildup of hydrogen peroxide at the root of hair follicles and that this is due in great part to a reduction in the natural antioxidant compound catalase. In addition to helping cleanse the scalp and follicles, onion juice also promotes the production of catalase. Catalase prevents grey hair as well as promotes thicker hair follicles. It is also shown to work for the skin condition vitiligo.
How-to: Divide your hair into four sections, then apply the onion juice on your scalp not on your hair. Massage gently and leave it on for 15 minutes till 1 hour, then rinse you hair well. Apply it daily and after 2 weeks ( at least) you may see great results. Shampoo afterwards to help remove the onion odor.
Some helpful links with more information on gray hair & baldness remedies: