Short answers, pulled from the story.
The first synthetic hair dye was created by Eugène Schueller, the founder of L'Oréal, in 1907. The chemical foundation for synthetic hair dyes was established in the 1860s with the discovery of para-phenylenediamine's reactivity with air.
Between 50 and 80 percent of women in the United States have reported using hair dye. At-home coloring in the United States reached $4.8 billion in sales in 2024 and is projected to rise to $8.2 billion by 2033.
Permanent hair color uses ammonia and a hydrogen peroxide developer to open the hair shaft and lock large dye molecules inside, making it resistant to washing out. Semi-permanent color contains no ammonia or developer, sits just under the cuticle layer, and washes out in roughly 4 to 8 shampoos.
The International Agency for Research on Cancer has concluded that some hair dyes and many chemicals used in the dyeing process can be considered mutagenic and carcinogenic. Prolonged use of permanent dark hair dyes has been associated with a doubled risk of some types of blood cancer, and a 2017 meta-analysis by Professor Kefah Mokbel found a possible 20 percent increased risk of breast cancer among users.
Balayage is a technique in which hair color is painted directly onto sections of hair without foils to contain the color. It generally costs more than ombré because it requires more precise hand-painting, and it produces a more natural-looking result.
The ancient Egyptian Ebers Papyrus, dated to around 1550 BCE, contains recipes for dyeing gray hair and eyebrows. Assyrian clay tablets from the 7th century BCE describe a formula using the gall of a black ox, cypress oil, licorice, and honey to turn gray hair black.