The germs and toxins that cause food borne illness do not change the smell or taste of the food. You cannot see if the food is contaminated. Only proper preparation, cooking and storage lowers the risk of foodborne illness. My raw hamburger I bought at the store was red on the outside, but gray on the inside. The retailer is hiding old meat inside fresh ground beef.
Red meat contains a pigment called oxymyoglobin.
The inside portion of the meat, while just as wholesome, may be darker due to the lack of oxygen. This is not an attempt to camouflage older ground beef. Sell-by dates are established by the stores based on a reasonable estimate of the time the product quality can be expected. This allows a few days for the consumer to store the product and use it or to freeze it for later use. For quality reasons, it is best to use or freeze the product immediately.