Hair colour depends on the presence of pigments within the hair shaft. Hair colour is formed from melanocytes. These stem cells working in conjunction with the hair follicle stem cells. The amounts of two types of melanins: eumelanins (brown and black pigments) and pheomelanins (red and yellow pigments) will determine the intensity of the colour. These pigments are produced within the hair shaft.
As we age it is normal for the colour to fade or lighten. The build-up of hydrogen peroxide causes melanocyte destruction and greying of the hair.
Scientific studies were carried out in the UK and Germany in 2009. Into the natural build-up of hydrogen peroxide, used as a product by people to lighten their hair. For the majority when younger, an enzyme called catalase breaks down hydrogen peroxide. Maintaining the darker pigmentation of the hair shaft.
Over time as we get older we have less of the enzyme catalase. As well as MSR A and B enzymes that repair hydrogen peroxide related damage to hair follicles. This combination of increasing levels of some and lower levels of others disturbs the level of melanin production.