EngineerGuy Bill Hammack explains that the finish is achieved by oxidizing—or rusting [Ed. note: yes, rust is the oxidization of iron, specifically, but the common usage is quite understood]—the aluminum on the surface, enough for dye to seep into the tiny holes created by the process. The metal seems painted, but the color actually becomes part of the aluminum. It’s pretty cool, and another way that Apple and other bright-and-shiny makers get creative with engineering when it comes to design.
The beauty of corrosion.