Hair coloring

Hair coloring is the cosmetic practice of changing or enhancing hair color with dyes, bleaches, or other coloring products. Methods range from temporary surface coatings to lasting chemical changes inside the hair shaft.

Hair follicle: A skin structure that produces a hair fiber and contains the cells that give growing hair its natural pigment. New hair grows from the follicle with pigment made by melanocytes; coloring products act on the fiber beyond it.
Permanent hair color: Hair color produced by chemical reactions that create lasting pigment within the hair fiber. It provides durable shade changes and gray coverage, but regrowth remains visible.
Hair shaft: The visible portion of a hair fiber, composed mainly of keratinized cells. Coloring products interact with the shaft’s outer cuticle and inner cortex.
Henna: A plant-derived dye made from Lawsonia inermis leaves, used to color hair and skin. Henna deposits color without the oxidative chemistry used by many permanent dyes.
Hair damage: Structural weakening or surface disruption of hair fibers caused by physical, chemical, or environmental stress. Repeated bleaching and dyeing can weaken fibers and increase dryness or breakage.
Melanin: A group of pigments that contribute color to hair, skin, and eyes. Hair dyes alter or cover the melanin that gives untreated hair its color.
Semi-permanent hair color: Hair color that deposits dye on or near the hair surface and gradually washes out. It adds tone without the same lasting chemical change as permanent color.
Hair cuticle: The outer layer of overlapping cells that protects the hair shaft. The cuticle affects how easily color enters and how quickly it fades.
Hair gloss: A sheer hair treatment that adds shine and may deposit a subtle tint or refresh existing color. Glossing can refresh tone with less dramatic change than full hair coloring.
Contact dermatitis: An inflammatory skin reaction caused by direct contact with an irritant or allergen. Some hair-color ingredients can trigger scalp or skin reactions, especially in sensitized people.

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