Why does a white sauce (bechamel,veloute) sometimes turn grey?
Cook ware is the number one item to look at.  Aluminum pots/pans will cause a greying to your sauce if they are not clad in stainless steel.  You'll get a reaction with acids and the aluminum that will cause disclouration to a light coloured sauce.

Aluminum gives great heat conduction for the dollar but is reactive.  That's why you'll see so many present day aluminum pans will be aluminum cored with a steel outer to stop that reaction but give the rapid heat transfer.