Discolored toenails are almost always caused by one of a handful of common issues: fungal infection, physical trauma, nail polish staining, or a skin condition like psoriasis. In most cases, the color itself tells you a lot about what’s going on. Yellow or white patches typically point to fungus, dark brown or black usually means a bruise, and green signals a bacterial infection. Here’s how to figure out what’s behind your nail changes and when the discoloration might be worth a closer look.
Fungal Infection: The Most Common Cause
Toenail fungus (onychomycosis) is far and away the most frequent reason for discolored nails, especially in adults over 60. The infection gets under or into the nail plate and produces visible changes that depend on where the fungus takes hold.
The most common pattern starts at the tip or side of the nail and works its way back toward the cuticle. You’ll notice the nail pulling away from the nail bed, thickening, and turning yellow, white, or brownish. The nail may become crumbly or develop a chalky buildup underneath. A less common type produces white, powdery patches across the surface of the nail that you can sometimes scrape off. A third type starts near the cuticle and moves outward, creating white discoloration closer to the base of the nail.
Fungus thrives in warm, moist environments, which is why toenails are affected far more often than fingernails. Risk factors include wearing tight shoes, sweating heavily, walking barefoot in communal showers or pool areas, and having athlete’s foot (the same types of fungi cause both). If only one nail is involved, fungus or trauma is the most likely explanation.
Bruising Under the Nail
Stubbing your toe, dropping something on your foot, or wearing shoes that press repeatedly against your nails (common in runners) can cause bleeding under the nail. This is called a subungual hematoma, and it’s one of the most dramatic-looking causes of discoloration.
The spot usually starts out red or purple and gradually turns dark brown or black over the following days. It doesn’t spread or change shape the way other conditions might. Instead, the discolored area slowly grows out with the nail. You’ll see more and more healthy nail appear at the base near the cuticle while the bruised section moves toward the tip. This process takes six to nine months for a toenail to fully clear, so patience is key. The nail may loosen or even fall off if the trauma was severe enough, but a new nail typically grows in behind it.
Nail Polish and Chemical Staining
If you regularly wear dark nail polish, especially reds, oranges, or deep shades, you’ve probably noticed a yellowish tint after removing it. Pigments in the polish can seep into the surface layers of the nail over time, particularly when polish is worn for weeks without a break or when no base coat is used. This discoloration is cosmetic and harmless. It grows out on its own once you give your nails a rest, though it can take several months given how slowly toenails grow.
Green Nails: A Bacterial Clue
A green or blue-green tint on a toenail almost always means a Pseudomonas bacterial infection. These bacteria produce a pigmented film on the nail surface, and the color is distinctive enough that it’s sometimes called “green nail syndrome.”
This infection tends to develop in nails that are already damaged or compromised, often by a pre-existing fungal infection, chronic moisture exposure, or a nail that has partially lifted from the bed. People whose hands or feet are frequently wet, such as dishwashers, janitors, healthcare workers, and hairstylists, are at higher risk. Artificial nails and prolonged use of nail products may also create conditions that favor the bacteria, though the evidence on this is limited.
Psoriasis and Other Skin Conditions
Nail psoriasis can closely mimic fungal infections, which makes it one of the trickier causes to identify on your own. But there are a few distinguishing features. Psoriatic nails often develop small pits across the surface, tiny shallow divots that look like someone pressed a thumbtack into the nail. You may also notice reddish-brown splotches underneath the nail, sometimes called “oil drop” spots because of their translucent, brownish appearance. These oil-drop spots don’t occur with fungal infections.
Another clue is the pattern of involvement. Psoriasis usually affects multiple nails on both hands and feet, often alongside skin symptoms like scaly patches on the elbows, knees, or scalp. If the nail lifts away from the bed, psoriasis tends to produce a reddish line at the border between the attached and detached portions. A single discolored toenail with no skin symptoms elsewhere is much more likely to be fungus or trauma than psoriasis.
When a Dark Streak Needs Attention
Most toenail discoloration is benign, but a dark streak running lengthwise along the nail deserves prompt evaluation. Subungual melanoma, a type of skin cancer that develops under the nail, can appear as a brown or black band running from the cuticle to the tip. It’s rare overall but accounts for a significant proportion of melanoma cases in people with darker skin tones, including Black, Asian, and Native American populations.
Dermatologists use a set of warning signs to distinguish melanoma from harmless pigmentation. A band wider than 3 millimeters with uneven or blurry borders is more concerning than a thin, uniform line. Color that bleeds beyond the nail onto the surrounding skin fold (known as Hutchinson’s sign) is a particularly important red flag. A streak that changes in width, color, or shape over time, or one that doesn’t resolve despite treatment for other conditions, also warrants a biopsy. Peak incidence is between ages 40 and 70.
A simple bruise can look similar at first glance, but bruises move forward with nail growth and eventually disappear. A melanoma streak stays fixed in place because the pigment-producing cells are at the base of the nail.
Yellow Nail Syndrome: Rare but Worth Knowing
Yellow nail syndrome is an uncommon condition where all or most nails turn thick, yellowish-green, and grow extremely slowly. What sets it apart is that it comes with other symptoms: swelling in the legs or feet from fluid buildup (lymphedema) and chronic respiratory problems like coughing, recurrent lung infections, or fluid around the lungs. If your nails are yellow but you don’t have breathing issues or unexplained swelling, this diagnosis is very unlikely. The condition has been associated with thyroid disease, rheumatoid arthritis, and certain cancers, though the exact mechanism isn’t well understood.
Keeping Your Toenails Healthy
Most toenail discoloration comes down to moisture, trauma, or infection, all of which are largely preventable. Wash your feet daily and dry them thoroughly, especially between the toes. Change your socks at least once a day, and choose breathable footwear that doesn’t crowd your toes. Keep your nails trimmed short and straight across. If you visit a nail salon, confirm that the facility is licensed and that all tools are sterilized between clients.
For runners or anyone in shoes for long hours, properly fitted shoes with enough room in the toe box can prevent the repetitive micro-trauma that leads to bruising and nail loss. If you wear polish regularly, consider giving your nails periodic breaks and always using a base coat to reduce pigment absorption into the nail plate.