rainbow.tex

\documentclass{article}
\usepackage{xcolor}
\usepackage[active,tightpage]{preview}
\setlength\PreviewBorder{2mm}

\makeatletter
\chardef\popen=`(
\chardef\pclose=`)
\catcode`(=\active
\catcode`)=\active
\newcount\clcount \clcount\m@ne
\def({\bgroup
  \advance\clcount\@ne
  {\select@color\popen}}
\def){{\select@color\pclose}\egroup}
\def\select@color{%
  \ifcase\clcount
    \color{red}\or\color{green}\or\color{blue}\or
    \color{cyan}\or\color{magenta}\or\color{yellow}\or
    \color{black}\or\color{gray}\or\color{white}\or
    \color{darkgray}\or\color{lightgray}\or
    \color{brown}\or\color{lime}\or\color{olive}\or
    \color{orange}\or\color{pink}\or\color{purple}\or
    \color{teal}\or\color{violet}\fi}
\makeatother

\begin{document}
\begin{preview}
$(\lambda a.(((\lambda x.(\lambda y.((xy)y)))g)((\lambda u.(ab)) f )))$
\end{preview}
\end{document}