\usepackage{tikz}

\definecolor{monOrange}{rgb}{0.97,0.35,0.04}

\newcommand*\commentterm[4][]{%
   \begin{tikzpicture}[anchor=base west,%
                       baseline,%
                       inner sep=0pt,%
                       outer sep=0pt,%
                       minimum size=0pt]%
      \node(xa){$#3$};
      \node[overlay,at=(xa),shift=(#2)](xb){#4};
      \draw[overlay,->,shorten <=2pt,shorten >=2pt,#1](xb)to(xa);
   \end{tikzpicture}%
}

$z=2^{\commentterm[color=blue]{\n:2cm}{\color{red}n}{\color{monOrange} \n:2cm}}$