Meat that has just turned grey is usually still safe to eat although not as appetizing.
The meat changes to a grey color due to a chemical reaction in the meat when it's not exposed to oxygen.
I've eaten ground beef as well as other meats including steak that had turned grey and it never caused any health issues.
As long as the meat does not smell bad or is not slimy feeling then the meat even when grey in color is safe to eat.
If it smells bad or the change in color of grey looks even worse throughout the meat then throw it out just to be safe.
But as long as it's just grey mostly on the surface of the meat and is not slimy or sour smelling then the meat is safe to eat.