Bleeding or bruising under an injured nail will cause a black or purplish appearance. You may need to have the blood drained from under the nail. The black appearance will most often go away as the injury heals, but this may take weeks.
Occasionally the black appearance under an injured nail may mean damage to the nail matrix, the area where the nail first begins to form. If this is the case, it may be necessary to remove the nail and repair the matrix.
Melanoma may give a black, irregular appearance to an uninjured nail.
Blue nails may occur as a side effect of a medicine. Blue nails are also caused by problems that reduce the amount of oxygen in the blood, such as asthma and COPD, severe anemia, cold exposure, exposure to high altitude, peripheral arterial disease, or shock.
Brown streaks occur normally in dark-skinned persons and are of concern only if they are new or changing. Brown streaks may also be caused by a medicine or malnutrition.
Pale white nails may be caused by nutritional disorders, such as anemia or zinc deficiency, or other medical problems.
White specks, spots, or bands (leukonychia) in the nail are common with mild injury. You may not even have been aware of the injury. These marks can last for weeks or months and go away on their own without any treatment.
Yellow nails occur when the nail separates from the nail bed (onycholysis) because of an injury, a skin condition, or an infection. It is also caused by medical problems such as chronic lung disease or cancer. Nails can also become yellow from smoking cigarettes or from using some nail polishes, especially red polish.