Short answers, pulled from the story.
Assyrian herbals from the 7th century BCE describe using gall of a black ox, cypress oil, licorice, and honey to turn gray hair black. These recipes appear on ancient clay tablets as some of the oldest known cosmetic preparations for hair dye.
Eugène Schueller created the first synthetic hair dye in 1907 while founding L'Oréal. This innovation followed William Henry Perkin's discovery of mauveine in 1856 and the development of para-phenylenediamine reactivity in the 1860s.
Root touch-ups apply color only to the most recent section of re-growth usually within the first inch nearest the scalp. These applications repeat every 4 to 6 weeks as natural color grows in and becomes apparent.
Synthetic permanent hair coloring requires three components including 1,4-diaminobenzene historically or 2,5-diaminotoluene currently along with a coupling agent and an oxidant. The process occurs under basic conditions involving oxidation of primary intermediates to quinone state.
Prolonged use of permanent dark hair dyes can double a person's risk of getting blood cancer types including leukemia and non-Hodgkin's lymphoma. Professor Kefah Mokbel published a meta-analysis in October 2017 showing hair dye use might increase breast cancer risk by 20% among users.