After cooking, most of the red has disappeared and the meat is mostly brown. One package that is a day or two older than the other may provide the contrast. You can also compare cooked and uncooked beef.
Fats combine with oxygen easily to develop strange flavors and odors.
This occurs because some of the fat has been oxidized to form chemicals that taste and smell bad. The special name for this is rancidity. If you leave a small amount of oil uncovered for a week or more before the session, you may be able to demonstrate the odor. Both freshness and packaging affect meat color. It changes to a brownish color when it has become completely oxidized or when microbes start to grow and spoil the meat.