A spot color is a one that is printed with its own ink while a process color is one that is printed using four inks (cyan, magenta, yellow, and black). As it stands one can have only process colors in a document produced by XeLaTeX. The spotcolor package by Jens Elstner is a tool that allows users of pdfLaTeX to have spot colors in their documents. Unfortunately, this package does not work with XeLaTeX and so I decided to “port” this package to XeLaTeX. I am using the term “port” because the package defines commands that use pdfTeX’s primitives that are not available to XeTeX. For instance, primitives like \pdfobj and \pdflastobj do not exist in XeTeX. Apart from this, I had noticed that in order to be able to color the background of a page using a spot color, I had to modify xdvipdfmx so to make it understand a new command. The xespotcolor package is the result of this task. The latest version of xdvipdfmx has included the patches that I submitted to the developers.
Update: People can no use the xespotcolor package with either XeLaTeX or LaTeX to create documents with spot colors.