A common technique used by CD artwork designers is to expose the silver background of the disc to give the silver foil effect.  The technical term is knocking out.  When you knock an area on the artwork essentially you don’t print anything on it.  To communicate such an idea you should have a separate layer on the artwork to represent the knockout.  Another term you may want to know is called a white flood.  By white flooding, the silver CD will be covered with a white background first.  Doing so will make the later printing as if it is printed on a piece of white paper.

Showing below are examples of knocking out, disc with a white flood, and disc without white flood.