I've read that myoglobin localization is responsible for the darker colour of leg muscles in turkeys. Why does this localization occur in terms of any of cell biology, molecular biology, or biochemistry? I'm not looking for bigger picture explanations at the levels of physiology or evolution, although I understand that they'll be connected to this question.
"Localization" is probably the wrong term, but I think I know what you mean... It more has to do with "expression." While the exact mechanism to my knowledge is not elucidated, it has to do with the type of motor neuron innvervating the fiber.
This has been demonstrated by studies where they cross-inervated; that is, they took the neurons which usually innervate Type I muscle fibers and innervated Type II muscle fibers with them and vice versa. The Type I fibers took on fast glycolytic properties, whereas the Type II took on the slow oxidative features (such as higher levels of mitochondria and myoglobin that you're talking about).
Sherwood L, Kell R. Human Physiology, First Canadian Edition. pp. 282-283.