It is possible to use pantone color in XeLaTeX. I have found only PdfLaTeX in Using Pantone/spot colors and TikZ together. I have both CMYK and Pantone color and must use XeLaTeX because many OTF Fonts are used in my book. How to do?
XeLaTeX – Using Pantone/Spot Colors
xetex
Related Solutions
For me your sample works just fine, with the line:
\setmainfont{Adobe Garamond Pro}
uncommented (and the previous one commented out).
I suspect the font isn't cached. Try running [sudo] fc-cache -fv
You can avoid the issue by selecting the page size to be equal to the picture size. Therefore you don't need preview and it works fine with XeLaTeX. For this you need to save the picture in a box register. Because geometry only allows to select the page size in the preamble you need to move the picture there, which is OK as long it inside a savebox (like inside a lrbox environment).
I'm planning to build a feature like this into standalone as an alternative to preview.
\documentclass[a4paper,12pt]{article}
\usepackage{tikz}
\usepackage{geometry}
%\usepackage[active,tightpage]{preview}
%\usepackage[pdftex,active,tightpage]{preview}
%\PreviewEnvironment[]{tikzpicture}
\pagestyle{empty}
\normalfont% required to select normal font already in the preamble
\begin{lrbox}{0}%
\begin{tikzpicture}[line width=1ex]
\draw (0,0) -- (3,1);
\filldraw [fill=red,draw opacity=0.5] (1,0) rectangle (2,1);
\end{tikzpicture}%
\end{lrbox}
\geometry{paperwidth=\wd0,paperheight=\ht0,margin=0cm}
\parindent=0pt
\begin{document}
\usebox{0}%
\end{document}
(The outer dark frame is the background of the PDF viewer)
The upcoming version 1.0 of standalone includes code similar to this and avoids the need of preview. It should work fine for XeLaTeX.
Best Answer
You can use the xespotcolor package. You need the latest version of xdvipdfm-x.