Myoglobin is a pigment protein found in muscle tissue that stores oxygen molecules until they are needed for metabolism. Pigment proteins are colored proteins found in hemoglobin in the blood and myoglobin in muscles.
When oxygen binds to myoglobin, it becomes oxy-myoglobin and turns bright red, but prolonged exposure to air (oxygen) causes the iron in the heme to oxidize, turning it into metmyoglobin and changing its color to brown. This is why fresh raw meat is bright red, but its color deteriorates over time.