It changes in response to the ammount of blood flowing at the surface of the skin and whether or not it is oxygenated. If blood is rich in oxygen, it is bright red. If it does not have oxygen (on the way to lung to exhale and reload it with oxygen on the inhale) it is a sort of blueish purple. The skin is naturally very pale and appears white, otherwise.

He apparently changes his head color at will, in a colorful display to attract the hens. Its part of the display, like his fan and strut.