what causes your fingernails to turn under
Fungus infections of the fingernails can also turn the nails brown or yellow. The texture usually also changes; nails often become very thick and may crack or peel. Nail fungus infections are more common in the toes than the fingers. They affect about 12 percent of Americans and may run in families.
When Toes Turn Dark. Nails change colors for multiple reasons. The most common culprit for a black toenail is blood under the hard tissue. Damage to the end of the toe causes blood to leak through the nail bed and stain the keratin a dark color. This is also known as a subungual hematoma, or a bruise under the nail.