There are two kinds of naturally occurring hair pigments: eumelanin and pheomelanin. Eumelanin generates a dark brown colour. Pheomelanin generates a red colour. Low concentrations of both eumelanin and pheomelanin result in blonde hair.
To mimic this blonde hair, the darker pigment containing more eumelanin must be removed leaving more of the lighter redder pigment, pheomelanin. Hydrogen peroxide reacts with melanin in eumelanin, but not with pheomelanin, as it is more stable and has a weaker reaction. The pheomelanin having a red hue often leaves bleached hair with an orange rather than blonde colour. In addition to the hydrogen peroxide, there are often persulfate salts that help speed up the process while stabilizers help prevent the breakdown of the hydrogen peroxide. The process can be damaging as hydrogen peroxide weakens the cell membrane and damages fats on the surface of the hair shaft. A pigment can then be applied to the lighter hair to change the colour/shade of the lightened hair.
Highlighting hair often colours only half the hair, cleverly requiring half the damaging hydrogen peroxide. A scalp hat where hair is pulled through and lightened allows bleaching nearer the root but is a painful process. The more modern alternative is the use of foils that section the hair and allow only partial colouring to take place.
Hair lightening is drying and damaging to the hair shaft and must be reapplied every 6 weeks to prevent dark roots from peeping through. The scalp may react to the harsh chemicals ad become red, flaky and itchy.