Fresh thawed ground beef should be a bright red or bright pinkish in color which means that it's fresh and good to eat.
However sometimes meat can turn a brown or grey color and still be perfectly safe to eat but it just means that it is not as fresh.
If the ground beef is turning really brown or has a bad smell to it or the ground beef is getting a slimy feeling to it then it's bad and should be thrown out.
But if the ground beef is just turning brown and is not smelling bad or doesn't have a slimy feeling to the ground beef then it can still be eaten but it just won't be as fresh.
A little brown or grey of the ground beef is okay but not when it turns really grey and really brown or has the bad smell or slimy feeling to it.