I have to also assume when they say "white meat" they mean light skin. So the cooked bird has a lighter appearance. Light meat vs dark meat comes from different muscle fibers (slow-twitch vs fast twitch).
"Muscles that receive sustained activity, like the wings and breast of ducks or geese, or the legs of chickens are called slow-twitch fibers. In order to keep activity at appropriate levels, the muscles need a constant supply of oxygen. Myoglobin is a protein that stores oxygen in the muscle cells and has quite a bit of pigment, resulting in a darker colored muscle."
There you have it.
"Muscles that receive sustained activity, like the wings and breast of ducks or geese, or the legs of chickens are called slow-twitch fibers. In order to keep activity at appropriate levels, the muscles need a constant supply of oxygen. Myoglobin is a protein that stores oxygen in the muscle cells and has quite a bit of pigment, resulting in a darker colored muscle."
There you have it.