Question: I purchased some ground beef yesterday and it was a bright red color. When I opened the package today, the center was more brownish-gray. It’s been refrigerated the whole time. Is it still safe?
As the U.S. Department of Agriculture points out, it’s common for the inside portion of packaged ground beef to become more brown or gray in color, even while the outside remains red.
The color difference you're noticing is usually caused by the lack of oxygen penetrating below the surface layer of the meat. When ground beef goes truly bad, on the other hand, other signs of spoilage are normally present, including an off odor, a sticky or tacky feel, or a slimy appearance.
As detailed here, raw ground beef can be safely refrigerated for one to two days. For longer-term storage, you should freeze ground beef no later than two days after purchasing it.