Stains on the nails

What usually explains a change in color in one or more nails and what are the signs that warrant a medical check-up?

Summary in 60 seconds

• Nail stains are not a diagnosis in themselves: they are a clinical sign.
• They can be white, yellow, brown, black, green, or reddish.
• The most common causes include microtrauma, manicures, fungus, and blood under the nail after a blow.
• They can also appear due to nail inflammation, such as psoriasis, or due to pigment (melanin).
• A new dark line on a single nail deserves special and priority attention, especially if it widens or stains the surrounding skin.
• Yellow and thick nails often suggest onychomycosis, but it should be confirmed before starting long treatments.
• Pinpoint white spots are usually more related to local trauma.
• Color is a guide, but the number of affected nails, shape, speed of change, and associated symptoms are also important.
• If the injury changes, hurts, bleeds, lifts the nail, or appears without a clear explanation, it is wise to consult a specialist.

What are they?

“Nail discoloration” is a common way to describe changes in the color of the nail apparatus. This change can occur on the nail plate, underneath it (in the nail bed), or even on the surrounding skin. From a medical perspective, the finding may correspond to leukonychia (white spots), melanonychia (brown or black spots), pigmentation due to infection, blood under the nail, or changes secondary to inflammation. Singal 2020 Iorizzo 2022

A stain on the nail is not a diagnosis: the color is indicative, but the medical history and examination are what really define the problem.

Why do they appear?

The causes are best understood by considering four main groups. The first is trauma : blows, repeated friction, nail biting, or aggressive manicures can leave white spots, lines, or blood under the nail. The second is infection , especially fungal infections, which often turn the nail yellow, white, or brown and also thicken or make it brittle. The third is inflammation , such as nail psoriasis, which can produce «oil droplet» spots, dots, blood lines, and partial detachment. The fourth is pigment , especially melanin, which can form a brown or black band; most of these bands are not melanoma, but some do require further investigation. Iorizzo 2022 Leung 2020 Singal 2020

In a smaller group of patients, the color can also change due to bacteria. The so-called “green nail” is usually associated with Pseudomonas , especially when the nail is detached or frequently remains moist. Much less often, some extensive white lesions or certain multiple splinter hemorrhages can be associated with systemic problems and warrant further evaluation. Haber 2016

What do they look like?

Small white spots are usually seen as isolated dots and, in many cases, are related to microtrauma or manicure habits. Diffuse yellow or whitish spots are often accompanied by thickening, brittleness, or subungual debris, suggesting a fungal infection. Brown or black spots can appear as a vertical band or a more diffuse area; sometimes they are due to benign pigmentation, and other times melanoma must be ruled out. Greenish spots suggest bacterial colonization. Reddish, purple, or dark brown spots that appear after a blow may correspond to blood under the nail and, over time, tend to move toward the free edge as the nail grows. Iorizzo 2022 AAD 2025 AAD 2021

In what diseases or conditions do they usually occur?

The following can be mentioned as a guide:

1. Very frequent

• Microtrauma and manicures : white spots or minor discoloration resulting from minor bumps, friction, nail biting, or cuticle manipulation. Iorizzo 2022
• Onychomycosis (fungal infection) : yellow, white, or brown nails, with thickening, brittleness, separation, or debris under the nail. Leung 2020 Frazier 2021
• Subungual hematoma : reddish, purplish, brown or black spot after a clear blow or repeated trauma.

2. Frequent

• Nail psoriasis : pitting, oil droplet-like spots, blood lines, peeling or brittle nails. AAD 2023
• Benign longitudinal melanonychia : a brown or black band, sometimes related to melanocytic activation, drugs, friction, or individual pigmentary characteristics. Singal 2020
• Splinter hemorrhages : fine reddish-brown lines, often caused by trauma, although they can also be seen in psoriasis and other inflammatory conditions. (Haber 2016)

3. Infrequent

• Green nail due to Pseudomonas : typical in moist, detached nails or nails with chronic paronychia. Chiriac 2015
• Leukonychia associated with systemic disease : more relevant when the white discoloration affects several nails or almost the entire nail, not as a simple isolated spot. Iorizzo 2022
• Subungual melanoma : rare, but clinically very important due to its diagnostic and prognostic impact. Darmawan 2022 AAD 2021

When should I consult?

Consult a dermatologist if you notice any of these signs:

• A new brown or black streak on a single nail , especially if it widens over time.
• Pigment that reaches the skin around the nail (for example, the cuticle)
• Pain, bleeding, lump, nail that splits or lifts without a clear cause.
• A dark spot that does not «move» outwards as the nail grows.
• Yellow, thick or brittle nails that last for months or are accompanied by pain.
• Green color with a bad smell, persistent moisture, or swelling around it.
• Several white or altered nails along with other general symptoms or known diseases.
• Diagnostic uncertainty : In nail conditions, delaying a clear diagnosis can postpone the correct treatment. AAD 2021 BAD 2026 Darmawan 2022

What NOT to do?

It’s unwise to assume that every yellow nail is «just a fungal infection» or that every black streak is caused by an injury. Aggressive scraping, piercing the nail at home, covering a new, changing lesion with polish or gel, or starting oral antifungals without a confirmed diagnosis are also not good ideas. In onychomycosis, several guidelines recommend confirming the diagnosis before prescribing systemic treatment, because other diseases can present very similarly. AAD 2025 Frazier 2021

Treating a yellow nail or a dark band «blindly» can delay the correct diagnosis.

What does a dermatologist usually do?

The first step is a focused medical history : when the spot appeared, if there was any injury, if it changes size, if it affects one or more nails, if there is pain, medications, psoriasis, previous infections, or frequent exposure to water. Then, examine all the nails , not just the most visible one. Singal 2020 Darmawan 2022

If the lesion is pigmented, dermatoscopy or onychoscopy can be used to assess the color pattern and determine whether it appears benign or suspicious. If a fungal infection is suspected, a nail clipping or sample is usually requested for examination. If the white pattern is extensive or affects many nails, additional tests are sometimes performed. And if there are signs suggestive of melanoma or another tumor, a nail biopsy may be necessary. Frazier 2021 Darmawan 2022 Iorizzo 2022

Frequently Asked Questions

Do white spots mean a calcium deficiency?

• This is not usually the primary explanation. In most isolated white spots, the most likely cause is minor trauma to the nail matrix or manicure habits. Iorizzo 2022

Is a black line on the nail always melanoma?

• No. Many dark bands are benign. Even so, a new band on a single nail that grows, widens, or pigments the surrounding skin should be evaluated as a priority. Singal 2020 AAD 2021

Are yellow nails usually caused by fungus?

• Often, yes, especially if the skin is also thick, brittle, detached, or has debris underneath. But psoriasis, trauma, and other causes can also mimic it, so confirmation is often necessary. Frazier 2021 AAD 2025

How long does it take for the white spots to disappear?

• If they are caused by minor trauma, they usually shift outwards as the nail grows. On fingers, this can take several months. Iorizzo 2022

Can a stain from a bump or knock look worrying?

• Yes. A subungual hematoma can appear very dark and be mistaken for other causes. Its progression helps: after the trauma, the discoloration usually moves toward the distal edge as the nail grows. DermNet, subungual hematoma

Why does a fingernail turn green?

• The typical cause is colonization by Pseudomonas , favored by moisture, onycholysis, or chronic inflammation around the nail. Chiriac 2015

Can I cover the stain with nail polish while I keep an eye on it?

• This is not ideal. To properly assess a pigmented lesion, it is important to be able to observe its actual evolution and decide whether further investigation is necessary. AAD 2021

Bibliography and sources

• Singal A, Bisherwal K. Melanonychia: Etiology, Diagnosis, and Treatment . Indian Dermatol Online J. 2020;11(1):1-11.
• Iorizzo M, Starace M, Pasch MC. Leukonychia: What Can White Nails Tell Us? . Am J Clin Dermatol. 2022;23(2):177-193.
• Leung AKC, Lam JM, Leong KF, Hon KL, Barankin B, Leung AAM, et al. Onychomycosis: An Updated Review . Recent Pat Inflamm Allergy Drug Discov. 2020;14(1):32-45.
• Frazier WT, Santiago-Delgado ZM, Stupka KC II. Onychomycosis: Rapid Evidence Review . Am Fam Physician. 2021;104(4):359-367.
• Darmawan CC, Ohn J, Mun JH, Kim S, Lim Y, Jo SJ, et al. Diagnosis and treatment of nail melanoma: a review of the clinicopathologic, dermoscopic, and genetic characteristics . J Eur Acad Dermatol Venereol. 2022;36(5):651-660.
• Haber R, Khoury R, Kechichian E, Tomb R. Splinter hemorrhages of the nails: a systematic review of clinical features and associated conditions . Int J Dermatol. 2016;55(12):1304-1310.
• Chiriac A, Brzezinski P, Foia L, Marincu I, Pinteala T, Solovan C, et al. Chloronychia: green nail syndrome caused by Pseudomonas aeruginosa in elderly persons . Clin Interv Aging. 2015;10:265-267.
• American Academy of Dermatology Association. How to check your nails for melanoma [Internet] . 2021 [citado 2026 abr 19] .
• American Academy of Dermatology Association. Nail fungus: Signs and symptoms [Internet] . 2025 [citado 2026 abr 19] .
• American Academy of Dermatology Association. Nail fungus: Diagnosis and treatment [Internet] . 2025 [citado 2026 abr 19] .
• American Academy of Dermatology Association. What is nail psoriasis, and how can I treat it? [Internet] . 2023 [citado 2026 abr 19] .
• British Association of Dermatologists. Fungal nail infections [Internet] . 2026 [citado 2026 abr 19]