Fresh ground beef should be a red in color which means it's at is freshness and it's best to cook and eat the ground beef while it's still red in color.
But when ground beef turns brown it can still be safe to eat and is mostly caused by a lack of oxygen which means that the ground beef is on it's way to going bad but it's not bad as in unsafe to eat.
The ground beef that has turned brown is usually just not as fresh as before and as long as it's not slimy or has a bad smell to it then the ground beef is perfectly safe to eat.
I've eaten brown ground beef before several times and never have gotten sick from it.