IColor Is Made Below the Skin, and Only Once

A hair is grown, not dyed. Beneath the surface of the scalp, each follicle ends in a rounded bulb, and inside that bulb two populations of cells work side by side. Keratinocytes divide and build the strand. Melanocytes, a smaller group positioned among them, make pigment and hand it across.

That handover is literal. Melanocytes extend thin, branching arms toward the cells forming the hair and deliver melanin packaged in small structures called melanosomes. The pigment comes to rest mainly in the cortex, the thick middle layer of the shaft, beneath a clear outer cuticle of overlapping cells. The cortex is built largely from keratin, the structural material of the strand, and what the eye reads as color is light passing into it and meeting the pigment held there.

Once the strand rises above the scalp, the cells inside it are no longer alive. Nothing new is added. This is the single most useful fact in the subject, because it explains so much of the rest. A scalp hair grows roughly a centimeter a month, and each centimeter is a record of what the follicle was producing at the time it was made.

It is also why hair color is better understood as a process than a property. The real question is never simply what color someone's hair is. It is what their follicles are making now, and in what proportion.

IITwo Melanins and a Fork in the Pathway

Both pigments begin with the same raw material. Inside the melanosome, the amino acid tyrosine is converted by an enzyme called tyrosinase into a compound named dopaquinone. Tyrosinase carries copper at its active site, which is why copper appears in almost every textbook account of pigment chemistry.

Dopaquinone is the fork. Where the sulfur containing amino acid cysteine is available inside the melanosome, the pathway turns toward pheomelanin, the red to yellow pigment, and the sulfur becomes part of its structure. Where cysteine is scarce, the pathway continues toward eumelanin, assisted by further enzymes including tyrosinase related protein 1 and dopachrome tautomerase.

The two products differ in more than color. Eumelanin granules tend to be larger, more ordered and more densely packed. Pheomelanin granules are smaller and less regular. Researchers describe the two as behaving differently under light as well, which is part of why red and blond hair can read so differently across a single summer.

The choice made at the fork is not random. It is set by a signal arriving at the surface of the melanocyte, and that signal is where genetics enters the picture.

IIIThe Signal That Chooses the Shade

On the surface of each melanocyte sits a receptor called the melanocortin 1 receptor, or MC1R. When a hormone called alpha melanocyte stimulating hormone binds to it, signaling inside the cell tilts production toward eumelanin. When that signal is weak, or when a competing molecule called agouti signaling protein occupies the receptor instead, production tilts toward pheomelanin.

Small differences in the MC1R gene change how readily the receptor responds. Most people with red hair carry variants of this one gene that dampen the receptor's response, which is why red hair is among the most closely studied traits in human pigment genetics, and why it so often travels with fair skin and freckling.

Red is unusual in being shaped so heavily by a single gene. Most of the range is not. A 2018 study led by researchers at King's College London, analyzing data from close to 300,000 people of European ancestry, identified 124 genetic regions associated with hair color, the large majority of them newly reported. Hair shade, in the general case, is the sum of many small contributions.

Those contributions can shift across a lifetime. Hair that is pale in early childhood commonly darkens through the school years and adolescence as eumelanin production rises. The genes have not changed. What the follicle is asked to make has.

IVPigment Keeps Time with the Hair Cycle

Follicles do not grow continuously. Each one moves through a cycle: a long growth phase called anagen, which on the scalp commonly runs for several years, a short transition called catagen, and a resting phase called telogen, after which the old hair is shed and a new one begins.

Pigment production is tied to that cycle. Melanocytes in the bulb produce melanin only during anagen. As the follicle enters catagen, the pigment making cells of the bulb are largely lost along with the lower follicle itself. When the next growth phase begins, the new bulb needs a fresh supply.

That supply comes from melanocyte stem cells, a reserve population held higher in the follicle in a region called the bulge. At the start of each cycle, some of these cells mature, travel down to the new bulb, and begin producing pigment. The color of every new hair depends on that reserve being present and able to respond.

A scalp follicle runs through this cycle many times across a lifetime, and at any moment its neighbors sit at different points in their own. It is this staggered, repeating renewal, rather than any single strand, that keeps a head of hair colored over decades.

VWhy Hair Turns Silver

The scientific name for graying is canities. In the plain terms set out above, it is what happens when a follicle begins a new growth phase and pigment production does not follow. The strand forms as before, with its cuticle and cortex in place, but without melanin inside it.

An unpigmented hair is not gray. Seen on its own, it reads as white or silver, because light entering it scatters through the keratin rather than being absorbed. Gray is something seen at a distance, the eye blending pigmented and unpigmented strands across a whole head. Salt and pepper remains the most accurate description of it.

Where the process happens inside the follicle has become clearer in recent years, mostly through work in mice. A 2023 study from NYU Grossman School of Medicine reported that melanocyte stem cells normally move between compartments of the follicle as it cycles, and that with repeated cycling many become stranded in the bulge, where they no longer mature into pigment producing cells. A 2020 study from Harvard University reported that in mice, acute stress acting through the sympathetic nervous system was associated with rapid depletion of the melanocyte stem cell reserve. Both were animal studies, and their authors described them as mechanisms to be examined further in people.

In people, the record is mostly observational. A 2012 survey in the British Journal of Dermatology of more than 4,000 adults across several countries tested the familiar rule that half of people are half gray by fifty, and reported the actual share at that age at roughly 6 to 23 percent, depending on the population. A 2016 study led by researchers at University College London, drawing on more than 6,000 people across Latin America, reported the first genetic variant associated with graying, in a gene called IRF4 that is also involved in melanin production. Studies described here were conducted independently and did not involve any Codeage product.

VIThe Strand After It Leaves the Follicle

Because the shaft is not living, its color can change only from the outside, and mostly in one direction. Sunlight is the clearest case. Ultraviolet light gradually oxidizes melanin in the cortex, which is why hair exposed through a summer often lightens toward the ends, where the strand is oldest and has seen the most light.

Water carries its own effect. Copper dissolved in pool water, often from pipes or from compounds used to keep pools clear, can bind to the hair surface and leave a faint green cast. It shows most on blond and silver hair, which have the least melanin to mask it. It is a deposit on the strand rather than a change in the pigment within.

Chemical coloring works on the same structure from another angle. Lightening products oxidize the melanin already present in the cortex, while dyes add color molecules into or onto the shaft. Neither reaches the follicle, which is why new growth at the root arrives in its own shade.

All of this returns to the distinction at the center of the subject. The strand records and weathers. The follicle decides.

VIIWhere This Fits

Hair is a structural material, built from keratin and anchored in tissue that renews in cycles across a lifetime, which is why pigment belongs in the Codeage library beside the structural biology beneath every strand and a multi-system reading of hair nutrition. It sits within Structural Integrity, the second pillar of The Longevity Code. A pillar names a location in a framework. It describes a category, not an outcome for any reader.

One distinction bears stating plainly, because this subject attracts claims that run ahead of the evidence. The molecules named in pigment chemistry, tyrosine and copper among them, describe what happens inside a melanocyte. They do not describe what adding any of them to a diet does to the color of hair, and Codeage does not present any formula as acting on it.

Hair is one of the few structures in the body where a biological process leaves a visible record, centimeter by centimeter, on a timescale anyone can observe. That is what makes it worth understanding plainly.