"hardness of the anodized aluminum oxide coating rivals that of a diamond, enhancing the abrasion resistance of the aluminum."
So.... the material is harder because it is anodized, but since the color is added after the anodizing process, it's susceptible to scratching off.
I have some anodized aluminum cookware that has held up very well. It has a darker "slate" color to it, but I didn't think it was added as a color -- I thought it was part of the metal, so that if you scratched it, you would just have more of the same color beneath the scratch. Too bad the iPhone 5 doesn't have the same color all throughout the material. I wonder what is done to achieve that vs. what is being done to the iPhone back.