In addition to turning yellowish, the nails may also change color to black, gray, brown, even to blue. The green-black nails can be caused by the growth of pseudomonas bacteria, the gray color can be caused by the consumption of drugs such as antimalarial or minocycline, and the color of chocolate can be caused by thyroid disease, pregnancy, or malnutrition. Blue nails can also be a sign of a liver condition disorder called Wilson disease, a condition in which the body has difficulty removing excess copper or when the body is deprived of oxygen.
