It does depend on the cause. When hair greys (loses color) as a person ages, it’s simply a result of the hair follicle having less/no melanin so that when new hair grows it doesn’t have color. However, there are other reasons hair can become gray. To some degree, aging is actually a build up of oxidative stress in the body. Oxidative stress is an imbalance between reactive oxygen species (ROS) that are created through normal cell functions and the body’s ability to remove the ROS through detoxification.   It is caused by many factors other than aging. Some of these are inflammation, some diseases, stress, impaired detoxification, poor diet (malnutrition) and environmental toxins. More recent studies are focusing on targeting oxidative stress in neurodegenerative diseases, Alzheimer’s disease and cancer. It does happen, on occasion, that when causes of oxidative stress are addressed that gray hair is ‘reversed’.