Since classicthesis
internally uses the tocloft
package, you can redefine \cftchapfont
and \cftchappagefont
to add the desired color; a little example:
\documentclass{scrbook}
\usepackage{xcolor}
\usepackage{classicthesis}
\renewcommand\cftchapfont{\normalfont\color{red}}
\renewcommand\cftchappagefont{\normalfont\color{red}}
\begin{document}
\tableofcontents
\chapter{Introduction}
\section{Test Section}
\subsection{Test Subsection}
\chapter{Another Chapter}
\section{Test Section}
\end{document}
An image of the resulting ToC:

classicthesis
are you using? Please add that information to your question.