Hair that gradually shifts toward reddish-brown tones is almost always caused by a change in the balance between two natural pigments in your hair, or by something external breaking down the darker pigment faster than the lighter one. Your hair contains two types of melanin: eumelanin, which produces brown and black tones, and pheomelanin, which produces red and yellow tones. When eumelanin is depleted, damaged, or diluted for any reason, the underlying pheomelanin shows through, and your hair looks warmer, more coppery, or reddish-brown.
The shift can happen for a surprising range of reasons, from sun exposure and pool water to mineral-heavy tap water, nutritional gaps, and hormonal changes. Here’s what’s most likely going on.
Sun Exposure Breaks Down Dark Pigment First
Sunlight is one of the most common reasons hair drifts toward reddish-brown, especially during summer. UV radiation in the 254 to 400 nanometer range oxidizes the structural proteins in your hair shaft, particularly the sulfur-containing molecules in keratin. But before those proteins take serious damage, your melanin pigments absorb the UV energy to protect them. That protective process gradually bleaches the melanin itself.
Eumelanin, the pigment responsible for dark brown and black tones, degrades more readily under UV light than pheomelanin does. As eumelanin breaks down, the reddish-gold pheomelanin that was always present becomes more visible. This is why brunettes often notice a warm, reddish cast after weeks in the sun rather than turning blonde. The effect is more pronounced in lighter hair, but it happens across all natural shades. Hair that’s already grown out of your scalp is essentially dead protein, so the sun damage is cumulative and won’t reverse on its own.
Chlorine Accelerates the Color Shift
If you swim regularly, chlorinated pool water can change your hair color even without sunlight. Research from TRI Princeton found that chlorinated water altered hair color in both the presence and absence of UV light, and that longer exposure produced more dramatic shifts. The combination of chlorine and sun exposure together caused the most significant changes in both natural brown and bleached hair.
Chlorine drives chemical reactions with melanin through multiple pathways, including oxidation and chlorination of the pigment molecules. These reactions reduce the electron structures that give melanin its dark color, effectively shifting the wavelength of light your hair absorbs. The result: brown hair takes on a warm, reddish, or brassy tone over time. If your pool has copper plumbing, the story gets worse. Dissolved copper can deposit on hair and create additional discoloration, sometimes even a greenish tint on very light hair.
Hard Water Deposits Minerals on Your Hair
The water coming out of your shower can quietly distort your hair color over months. Hard water, which is especially common in areas with limestone bedrock or older city pipes, contains high levels of calcium, magnesium, and sometimes iron, copper, and manganese. These minerals build up on the hair shaft with every wash, creating a film that changes how light interacts with your natural color.
Iron and copper are the biggest culprits for reddish shifts. These metals can oxidize on the hair surface and produce warm, brassy tones, particularly in lighter or color-treated hair. Manganese deposits tend to darken or stain. If your hair feels rough, looks dull, and has slowly taken on a reddish or orange cast that doesn’t match your roots, hard water buildup is a strong possibility. A chelating or clarifying shampoo designed to strip mineral deposits can help confirm this: if one wash noticeably changes the tone, minerals were likely the issue.
Nutritional Deficiencies Can Reduce Dark Pigment
Your body needs specific raw materials to produce eumelanin, and when those run low, your hair can lighten or shift in color. The amino acid tyrosine is a key building block for melanin synthesis. During periods of poor nutrition or chronic illness, your body may redirect tyrosine and related amino acids toward more urgent tasks like producing immune proteins, leaving less available for pigmentation.
Iron plays a direct role in this process too. A case study documented in PubMed described a teenager with iron-deficiency anemia whose black hair developed alternating bands of light and dark color. After iron supplementation, eumelanin production increased and the hair returned to its normal shade as it grew out. Copper also supports the enzyme that drives melanin production, so significant copper deficiency can reduce pigmentation as well. These nutritional causes are more common in people with restrictive diets, heavy menstrual periods, or chronic digestive conditions that impair nutrient absorption.
In severe childhood malnutrition, a visible pattern called the “flag sign” can appear: alternating stripes of lighter and darker hair running parallel to the scalp, each stripe corresponding to a period of adequate or inadequate nutrition. While this extreme presentation is rare in well-nourished populations, milder versions of the same mechanism can produce subtle color shifts.
Thyroid and Hormonal Changes
Thyroid hormones directly influence hair follicle cycling and pigmentation. They stimulate the production and distribution of melanin, so when thyroid function drops (hypothyroidism) or becomes dysregulated, melanin output can change. Some people with thyroid disorders notice their hair becoming lighter, warmer in tone, or more brittle. In rare cases, thyroid conditions can cause noticeable darkening or lightening of hair color.
Hormonal shifts during pregnancy, puberty, and perimenopause can also alter the ratio of eumelanin to pheomelanin your follicles produce. Many people report their hair color changing gradually during these transitions without any obvious external cause. These shifts happen at the follicle level, meaning you’ll typically notice the change at your roots first, with the new tone growing in over several months.
Your Genetics May Be Catching Up
Hair color isn’t permanently fixed by your genes at birth. It’s controlled in part by the MC1R gene, which acts like a switch on the surface of pigment-producing cells. When this receptor is fully active, your melanocytes primarily produce eumelanin (dark pigment). When it’s partially inactive due to common genetic variants, the balance tips toward pheomelanin (red pigment).
Many people carry one copy of an MC1R variant without knowing it. This can produce auburn or strawberry undertones that become more visible as other factors (sun, aging, hormonal changes) reduce overall eumelanin production. If one of your parents or grandparents had red or auburn hair, you may carry a variant that’s only now expressing itself. It’s also completely normal for hair color to shift naturally between childhood and adulthood as gene expression patterns change.
How to Correct Reddish-Brown Tones
The right fix depends on what’s causing the shift. If sun and chlorine are the likely culprits, the damage is in the existing hair shaft and won’t wash out on its own. A blue-pigmented shampoo is the most effective at-home option for neutralizing orange and red tones in brunette hair. Blue sits opposite orange on the color wheel, so it counteracts warmth directly. Purple shampoo, by contrast, targets yellow tones and won’t do much for reddish shifts.
For hard water buildup, a chelating shampoo or a dedicated mineral-removing treatment can strip deposits from the hair surface. Using one every week or two can prevent the gradual color distortion from returning. If you’re on well water or know your area has hard water, a shower filter that reduces dissolved metals is a longer-term solution.
If the reddish tone is growing in from the roots rather than appearing along the length of your hair, the cause is internal: hormonal, nutritional, or genetic. Checking your iron and ferritin levels through a simple blood test is a reasonable starting point, especially if you also have fatigue, brittle nails, or heavier periods. Iron supplementation has been shown to restore normal eumelanin production once levels recover, though it takes several months of new growth before the color change becomes visible.