Discolored toenail. You should see a dermatologist. The doctor will evaluate the type of coloration. Is it related to a medicine that you take (usually several nails). Is it a surface pigment from socks that rub with a shoe on only one toe? Is it a sport injury such as joggers toe with subtle bruising, or is it a pigmented streak underneath the nail that can be a serious problem. Please see your doctor right away.
Nail fungus . If a toe nail had turned a grey color, it usually means there is a fungal infection of the nail. There are several medications that may treat the problem. Consult your physician about treatment options.