Some hair dyes are basically fine-grained paint. But dyes that stick with you for more than a couple weeks physically and chemically change each hair. Learn how with HowStuffWorks.

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This is not my natural hair color. There are three basic chemical formulations of hair dye: temporary, semi-permanent, and permanent. Before we look at what each of them do, let’s look at the physical structure of hair.

Hair is dead stuff. Three layers of slightly different dead stuff. The core, called the medulla, is not pertinent to our interests today.

But it’s surrounded by a thick layer of cells called the cortex – no relation to your brain’s cortex. It’s where you find the pigmented melanin proteins that give hair its color.

Protecting the cortex is hair’s outermost layer: the cuticle. As hair’s armor, it’s made up of overlapping scales. Temporary dye just sticks to the cuticle – it’s more like paint, really, and it’ll usually all circle the drain with your next shampoo.

Semi-permanent dye contains molecules of pigment so tiny that they can slip between the scales of the cuticle and stick to the cortex. But it’s still more paintlike – it doesn’t chemically react with anything in the hair.

The wee pigment particles will wash back out through the cuticle’s scales with soapy water, so a semi-permanent dye lasts about 12 shampoos max.

Permanent dye, as the name suggests, is designed to stay with your hair until the hair grows or falls out. In general, permanent dye consists of two solutions. First, an alkaline chemical plus two types of particles that will come together to form the new color: dye precursors and dye couplers.

Second, the developer: an oxidizer, usually a weak solution of hydrogen peroxide. You (or your salon professional) mix ‘em together right before applying them to your hair.

The alkaline chemical (either ammonia or a gentler substitute) goes to work opening up the cuticle. For dye to be most effective, it needs access to hair’s cortex. The alkaline stuff temporarily softens and relaxes the cuticle’s scales.

The next hurdle in achieving a new hair color is getting rid of some of the existing melanin in the hair’s cortex. Enter the developer. It oxidizes the melanin molecules, breaking melanin’s double carbon-carbon electron bonds and giving up one of its own oxygen atoms to fill in the space.

The result: the melanin turns colorless and releases sulfur atoms. That’s right: part of permanent dye’s characteristic stink isn’t actually the dye at all, but an element of your hair passing into the air.

But that’s not the developer’s only job: It also kicks off the reaction that brings together the new color molecules by oxidizing the dye precursors. These are usually colorless chemicals that develop color when oxidized.

The resulting pigmented particles (called intermediates in industry lingo) are monomers that, left to their own devices, would slip through the cuticle’s scales like semi-permanent dye.

But the dye couplers react with the intermediates to form polymers of pigment that’re too big to just slip back out. That’s how permanent color resists fading through multiple washes: It’s trapped beneath the cuticle.

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Tro, Nivaldo. “Chemistry in Focus: A Molecular View of Our World”. Cengage Learning. Jan 1, 2015. p. 369.

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