Gray color model not working

You need to use the gray name in lower case and 0.1 instead of 10 (if you mean percent)

See here : https://en.wikibooks.org/wiki/LaTeX/Colors

This should work:

\documentclass[a4paper, 12pt,svgnames]{article}

\usepackage{xcolor}

\begin{document}

\textcolor[gray]{0.1}{text}

\end{document} 

Actually hyperref (or more precisely hycolor/xcolor-patch.sty) contains a patch for this

\documentclass{book}
\usepackage{xcolor}
\usepackage{hyperref}
\pagestyle{empty}
\begin{document}
\textcolor[Gray]{10}{text}
abc
\end{document}