\documentclass{article} % Herbert Voss
\usepackage{xcolor}
\makeatletter
\renewcommand\section{\@startsection {section}{1}{\z@}%
   {-3.5ex \@plus -1ex \@minus -.2ex}%
   {2.3ex \@plus.2ex}%
   {\color{red}\normalfont\Large\bfseries}}
\makeatother

\begin{document}
\tableofcontents
\section{foo}
bar 
\subsection{baz}
bar 

\end{document}