http://www.essortment.com/all/fingernailsnutr_rjbo.htm

"Discolored nails: Diabetes, stress, allergies and simple illness can cause your nails to appear discolored. A greenish nail color, however, can be a sign of infection, either in the nail bed or in your system.

Bluish nail beds can be a sign of lung trouble, such as emphysema or even asthma. A simple dark blue line in the nail can be a sign of skin cancer. Tiny black streaks can indicate a heart problem, while reddish-brown spots can indicate a deficiency of folic acid, protein or vitamin C.

Yellowing nails are early signals of various internal disorders, such as diabetes, respiratory or liver problems. White lines in or across the nail can signal fever, liver or heart disease, kidney disorders or, more likely, a lack of iron or zinc in your diet."