Short answers, pulled from the story.
Eugène Schueller created the first synthetic hair dye in 1907. This innovation followed the 1856 discovery of mauveine by English chemist William Henry Perkin and the 1860s discovery of para-phenylenediamine reactivity with air.
Permanent hair color generally contains ammonia and must be mixed with a developer or oxidizing agent to change hair color. The process uses 1,4-diaminobenzene or 2,5-diaminotoluene as primary intermediates along with coupling agents and hydrogen peroxide as the oxidant.
Root touch-ups are repeated every 4 to 6 weeks as the natural color grows in and becomes apparent. This maintenance is necessary for people who color their hair to disguise gray or maintain a solid level of coverage.
Hair coloring can result in adverse effects including temporary skin irritation, allergy, hair breakage, and skin discoloration. Some studies link prolonged use of permanent dark hair dyes to an increased risk of certain blood cancers and bladder cancer.
Fifty to 80 percent of women in the United States, Europe, and Japan have reported using hair dye. At-home coloring in the United States reached sales of 1.9 billion dollars in 2011 and were expected to rise to 2.2 billion dollars by 2016.
Balayage requires more precise hand-painting of hair sections and typically costs more than ombré. Ombré is a technique where hair is dark on the crown and becomes lighter toward the ends, while balayage is an off-the-scalp method that does not use foils.