Nail discoloration refers to the changes in the natural color of the nails. Normal healthy nails are usually strong, even, and pink in color. Your nails can become yellow, greenish, or stained due to a variety of reasons. The discoloration of the fingernails and toenails is mostly caused by the fungal infection. Conditions like pseudomonas bacterial nail infection, ingrown nail, yellow nail syndrome, and psoriasis, an autoimmune disease, can also result in nail discoloration. Frequent applying of nail polish is another cause that may lead to the yellowing of the nails. Sometimes, nail discoloration can be a sign of nutritional deficiency or an underlying medical problem.