Paladin - read this:
http://en.wikipedia.org/wiki/Temperi...mpering_colors
What is happening is a thin layer of oxide is forming on the surface, and the temperature at which that layer is formed determines what wavelength of light can pass through the film and then reflect back off the metal surface. Which is why a good polish is critical. And which is also why the finish is not durable - it doesn't take much to rub that thin surface layer off. As the saying goes, a color case hardened finish is only as good as its clear coat.