How can you tell when raw beef is bad?
As raw beef starts to turn bad it will begin to discolor, you will see various shades of pastel green and yellow, as it gets worse the discolored areas will begin to get slimy and you will notice a very bad smell.
'Color of Beef'
Beef muscle meat not exposed to oxygen (in vacuum packaging, for example) is a burgundy or purplish color. After exposure to the air for 15 minutes or so, the myoglobin receives oxygen and the meat turns bright, cherry red.
After beef has been refrigerated about 5 days, it may turn brown due to chemical changes in the myoglobin. Beef that has turned brown during extended storage may be spoiled, have an off-odor, and be tacky to the touch."