See a Podiatrist. A fungal nail infection can be very difficult to cure. Over the counter treatments are usually not very effective. Prescription medications are available. Oral medication called Lamisil (terbinafine) is often effective for toenail fungus. Blood tests must be done to monitor liver function. It is a generally a safe and effective treatment for many people.
Antifungals. If the discoloration is caused by a fungus using a topical antifungal will kill the fungus and the discoloration will disappear. It will take a while for the nail to grow out, as it does the discoloration will grow to the end of the toe and then be gone. There are many topicals medications, they all seem to be equally effective. Vicks vaporub is what I am recommending to my patients .