Noticing that only one nail has changed color can be unsettling, especially when the rest of your nails look normal.
The answer to why is one nail discolored often depends on whether the change came from trauma, infection, pigmentation, or a medical condition—and the pattern matters.
What Nail Discoloration Can Look Like
Nail discoloration can appear as yellowing, whitening, brown streaks, green tinting, blue-gray tones, or a dark spot under the nail.
It may affect the nail plate, the nail bed, or the skin around the nail, and the texture may also change at the same time.
When only one nail is involved, localized causes are more likely than whole-body causes.
That said, a single discolored nail should not be ignored if the change is persistent, spreading, painful, or associated with thickening, splitting, or bleeding.
Common Reasons One Nail Becomes Discolored
1. Trauma or repeated pressure
One of the most common explanations is injury.
A direct hit, tight shoes, aggressive manicures, or repetitive friction can cause bruising beneath the nail, known as a subungual hematoma.
Fresh trauma often looks red, purple, or black, then gradually grows out as the nail grows.
Minor trauma can also alter nail growth temporarily, leading to white spots, ridges, or a darker band.
Toenails are especially prone to discoloration from shoe pressure and running-related stress.
2. Fungal nail infection
Onychomycosis, the medical term for fungal nail infection, commonly causes yellow, white, or brown discoloration in one nail before it spreads.
The nail may become thick, brittle, crumbly, or lifted from the nail bed.
Fungal infections are more common in toenails because warm, moist environments help fungi grow.
They can also follow minor trauma, because damaged nails are easier for organisms to invade.
3. Bacterial infection
A bacterial infection can discolor a nail, especially if the nail has separated from the nail bed or if the surrounding skin is inflamed.
One well-known example is Pseudomonas, which can produce a green or green-black tint.
Bacterial nail infections may occur after prolonged moisture exposure, artificial nails, or repeated contact with water and cleaning products.
Swelling, pain, drainage, or odor may point toward infection rather than simple staining.
4. Subungual hemorrhage
Bleeding beneath the nail can create a dark red, purple, or black discoloration.
This usually follows trauma, but sometimes the injury is so minor that the person does not remember it.
A key clue is that the color grows outward with the nail over time.
If the dark area does not move as the nail grows, or if it appears without any injury, it should be assessed.
5. Pigment changes and melanonychia
Brown or black discoloration can come from melanin, the pigment that also colors skin and hair.
This is called melanonychia.
It may appear as a streak, band, or diffuse darkening in one nail.
Causes include benign pigment activation, certain medications, nail trauma, and, more rarely, melanoma under the nail.
A new dark band in a single nail deserves prompt evaluation, especially if it widens or changes shape.
6. Nail psoriasis and inflammatory skin disease
Psoriasis can affect nails and produce yellow-brown discoloration, pitting, thickening, and separation from the nail bed.
It may affect one nail more than another, even though psoriasis is often a multi-area condition.
Other inflammatory diseases, including eczema and lichen planus, may also alter nail color or texture.
These conditions often come with skin findings elsewhere on the body.
7. Staining from chemicals or cosmetics
Nail polish, artificial nails, dyes, tobacco smoke, and some cleaning chemicals can stain one nail more than others.
This is especially common if one nail has a rough surface, a previous injury, or prolonged exposure to a product.
Cosmetic staining usually stays on or near the surface and improves as the nail grows out or after removing the source.
It is less likely to cause pain, swelling, or structural changes.
When One Discolored Nail May Signal Something Serious
Most cases are benign, but certain patterns need medical attention.
A dark streak that is new, widening, irregular, or affecting only one nail should not be assumed to be harmless.
Seek evaluation sooner if the discoloration is accompanied by any of the following:
- Pain, swelling, or warmth around the nail
- Drainage, odor, or visible pus
- Nail lifting, crumbling, or major thickening
- A dark band that extends onto the cuticle or surrounding skin
- Bleeding without clear injury
- Rapid change in color or shape
- A pigment change that does not grow out over time
One important concern is subungual melanoma, a rare type of skin cancer under the nail.
It is uncommon, but it can present as a single dark streak or patch, especially in one thumb, big toe, or fingernail.
How Doctors Evaluate a Discolored Nail
A clinician usually starts with a physical examination and questions about recent injury, footwear, nail care, medication use, and exposure to moisture or chemicals.
They may also ask whether the discoloration is changing and whether other nails or skin areas are affected.
If infection is suspected, the nail may be clipped or scraped for lab testing.
If a pigmented lesion is concerning, a dermatologist may use dermoscopy or recommend a biopsy to rule out melanoma.
Photos taken over time can help show whether the discoloration is moving outward with nail growth, which often suggests trauma or staining rather than a deeper lesion.
What You Can Do at Home
If the nail is not painful and there are no warning signs, simple observation may be reasonable while the nail grows.
Fingernails usually grow faster than toenails, so changes often become clearer over weeks to months.
- Keep the nail trimmed and clean
- Avoid picking, scraping, or covering the nail with harsh products
- Wear well-fitting shoes to reduce repeated trauma
- Stop using new cosmetic products if staining began after application
- Dry hands and feet thoroughly after washing
Do not try to drill, puncture, or aggressively clean under the nail.
That can worsen injury or introduce infection.
How to Tell If It Is Growing Out
One useful clue is the nail’s natural growth pattern.
If the discolored area starts near the base and gradually moves toward the tip, trauma or temporary staining becomes more likely.
If the color stays fixed in place, becomes broader, or develops irregular edges, it may be arising from the nail matrix or nail bed and should be examined.
Because toenails grow slowly, this process can take months, which is why documentation with photos can be helpful.
Key Differences Between Common Causes
- Bruising: sudden onset after injury, often red, purple, or black
- Fungal infection: yellow, white, or brown discoloration with thickening or crumbling
- Bacterial infection: green tint, often with moisture exposure or nail lifting
- Melanonychia: brown or black streak or band, especially if new or changing
- Cosmetic staining: surface-level color change without pain or nail damage
Understanding the surrounding symptoms is often more useful than the color alone.
Nail texture, location, growth pattern, and whether only one nail is affected all help narrow the cause.
When to See a Dermatologist
A dermatologist should evaluate a discolored nail if the cause is unclear, if the change has lasted more than a few weeks, or if the nail has a suspicious dark stripe.
This is especially important when there is no history of trauma or when the discoloration involves the nail fold or surrounding skin.
Early assessment is also wise for people with diabetes, poor circulation, immune suppression, or a history of skin cancer, because nail infections and other nail disorders can be harder to treat and may progress faster.