There is no warranty on exhaust pipes and mufflers with regard to any discoloration. Discoloration (blueing) is caused by tuning characteristics, cam timing, carburetor jetting, overheating, etc. and is not caused by defective manufacturing. See Hardware, Chemicals & Tools for products that will help to prevent or remove blueing.
Some products may not be legal for sale or use on pollution controlled vehicles, check your local laws and the manufacturers' information.