According to a recent patent application, Apple is playing with the idea of dual anodization of their metals, presumably for their aluminum laptops and other metal computers. Anodization is a process of treating a metal to guard against corrosion (like rust).
The dual anodization process is designed as a way to give a metal different decorative properties, such as color, according to the patent application. The description below describes it in more detail:
A metal surface treated to have two anodized layers or regions may be used in electronic devices. The surface treatment may include performing a first anodization process to create a first anodized layer, removing the first anodized layer at select locations, and performing a second anodization process to create a second anodized layer at the select locations. The first and second anodized regions may have different decorative properties, such as color, and different structural properties, such as degree of abrasion resistance. One of the anodization processes may be hard anodization and the other may be standard anodization.
When we read about giving different decorative properties like color to different locations, we can't get the thought of polka dot MacBook Pro out of our imaginations. Though we're sure there's a more practical use for it.