When raw meat changes to a brownish-grey colour it’s usually because it's been exposed to oxygen.
Exposure to oxygen changes the colour of the meat. It's a normal part of the meat aging process and not necessarily a sign of spoilage.
To prevent it from happening keep your meat frozen – or stored in a vacuum pack in the fridge – till you're ready to cook it.
But... if the surface of the meat is slimy and sticky or if it smells “off”, it could be a sign of bacterial growth – so you're advised to play safe and bin it.