Coolant Discoloration:
The current Motorcraft Cooling System Stop Leak Pellets, service part number VC-6, used in both production and service, cause the yellow-colored Motorcraft Premium Gold Engine Coolant to change color ranging from a light golden brown to dark brown, depending upon the amount of pellet material in the coolant reservoir. This discoloration does not adversely affect the coolant or the vehicle’s cooling system components. No repairs are required for this condition. To avoid confusion with regard to coolant contamination, a new formulation of stop leak pellets will be implemented in production and service in the near future.
http://www.revbase.com/BBBMotor/TSb/DownloadPdf?id=116643