It has to do with the surface of the coins. On a microscopic level, the surface of the coins aren't smooth but are comprised of jagged ridges basically. When a coin tones, a silver oxide layer builds up on the coin...light then refracts through this layer and off surface ridges creating the colors. The surface ridges of a Morgan Dollar are larger that those of a Peace Dollar (basically, a Peace Dollar is much smoother on a microscopic level). This changes the visual properties of the toning. It's the same reason why a Morgan Dollar and a Peace Dollar have a different luster pattern.