Many causes for discolored nails: infection n(bacterial, viral, fungal),frostbite, auto-immune disease, trauma, neoplasia (cancer), and bizarre breed specific diseases.
I would make an appointment with your veterinarian to gather further information about the color change.
Your vet will pursue history, current drug use and diet along with a physical exam and possibly diagnostics to find an answer. There are too many possibilities for me to give you a simple answer.
Dr. Van Lienden
Dr. Raymond Van Lienden DVM
The Animal Clinic of Clifton
12702 Chapel Road, Clifton
Virginia, U.S.A. 20124