The color of a cut of poultry is determined by the type of muscle tissue within it.
Dark meat consists of slow-twitch muscles, which are used for extended periods of activity. In chickens and turkeys, this corresponds to the leg and thigh, which are constantly used for standing and walking. Dark meat derives its red (raw) or brown (cooked) color from myoglobin, an oxygen-carrying protein that can provide sustained energy to cells.
The breast and wings contain mostly white meat. White meat is made of fast-twitch muscles, which are better suited to short bursts of activity. Glycogen, which is used to store energy in fast-twitch muscles, is transparent when raw, and whitish when cooked.
Dark meat has more niacin, zinc, iron, riboflavin, thiamin, and vitamins B6 and B12 than white meat. White meat has slightly less fat and fewer calories than dark meat.