Color changes can occur in frozen foods. The bright red color of meat as purchased usually turns dark or pale brown depending on its variety. This may be due to lack of oxygen, freezer burn or abnormally long storage. Freezing doesn't usually cause color changes in poultry. As the U.S.
Department of Agriculture points out, it’s normal for fresh meat to change color during refrigerator storage. For instance, it’s common for beef to turn more of a brownish shade, due to oxidation. When raw meat goes truly bad, on the other hand, other signs of spoilage are usually present, including an off odor, a sticky or tacky feel, or a slimy appearance.