A pigment in steak (and other meat) called myoglobin gives it a dark red or purplish color. Exposure to oxygen transforms myoglobin into oxymyoglobin, which imparts more of a cherry hue to meat. To our eyes, that color equals delicious, so stores use oxygen-permeable plastic wrap to help it hold the cherry color.
Soon, though, prolonged exposure to oxygen and light changes oxymyoglobin into metmyoglobin. This is when steak starts to turn grayish-brown due to oxidation thanks to iron in the myoglobin.