\documentclass{testsuite}
\usepackage{fontspec}
\usepackage{xcolor}
\begin{document}
{\fontspec[Color=5500FF]{Verdana}Test}

{\fontspec[Color=red]{Verdana}Test} vs
{\color{red}\fontspec{Verdana}Test}

\definecolor{Foo}{rgb}{0.3,0.4,0.5}
{\fontspec[Color=Foo]{Verdana}Test} vs
{\color{Foo}\fontspec{Verdana}Test}
\end{document}