\input{umtest-preamble}
\usepackage{unicode-math}
\setmathfont[script-font     =      {Asana Math},
             script-features =      {Style=MathScript,Colour=FF0000},
             sscript-font    =      {Cambria Math},
             sscript-features=      {Style=MathScriptScript,Colour=0000FF}]
            {XITS Math}
\begin{document}
\[123456789^{123456789^{123456789}}\]
\end{document}
