Red meat is red because the muscle fibers which make up the bulk of the meat contain a high content of myoglobin and mitochondria, which are colored red.
Myoglobin, a protein similar to haemoglobin in red blood cells, acts as a store for oxygen within the muscle fibers.
Mitochondria are organelles within cells which use oxygen to manufacture the compound ATP which supplies the energy for muscle contraction. The muscle fibers of white meat, by contrast, have a low content of myoglobin and mitochondria.
The difference in color between the flesh of various animals is determined by the relative proportions of these two basic muscle fiber types.
The fibers in red muscle fatigue slowly, whereas the fibers in white muscle fatigue rapidly. An active, fast-swimming fish such as a tuna has a high proportion of fatigue-resistant red muscle in its flesh, whereas a much less active fish such as the plaice has mostly white muscle.
The color of meat is governed by the concentration of myoglobin in the muscle tissue which produces the brown coloring during cooking.
Chickens and turkeys are always assumed to have white meat, but free-range meat from these species, especially that from the legs, is brown.
This is because birds reared in the open will exercise and become fitter than poultry grown in restrictive cages.
The fitter the bird, the greater the ease of muscular respiration, and hence the increased myoglobin levels in the muscle tissue, making the meat browner.
All beef is brown because cattle are allowed to run around in fields all day, but pork is whiter because pigs are lazy.