Hi Greetings. Reversing grey hair to black is indeed among the tough results to show to a patient since this is a multifactorial problem. Since in most it's genetically mediated, it's tough to reverse or treat. However there are certain peptides and newer molecules application of which may be worth a trial. It's also important to rule out hormonal irregularities and nutritional deficiencies. Increase intake of oral anti oxidants. Tobacco smoking can accelerate greying.Meet a qualified dermatologist near you for further advise. Hope this helps