customizing chapter title format in classicthesis

Question

I'm using classicthesis and I'd like to change the formatting of the chapter title a little bit (f.e. size and color). I think I've found the according part in the classicthesis-style

% ********************************************************************
% layout of the chapter-, section-, subsection-, subsubsection-,
% paragraph and description-headings
% ********************************************************************             
\RequirePackage{titlesec}
% chapters
\ifthenelse{\boolean{@linedheaders}}%
{% lines above and below, number right
\titleformat{\chapter}[display]%             
    {\relax}{\raggedleft{\color{halfgray}\chapterNumber\thechapter} \\ }{0pt}% change color to red
    {\titlerule\vspace*{.9\baselineskip}\raggedright\spacedallcaps}[\normalsize\vspace*{.8\baselineskip}\titlerule]% change size to \huge
}{% something like Bringhurst  
\titleformat{\chapter}[display]%
    {\relax}{\mbox{}\oldmarginpar{\vspace*{-3\baselineskip}\color{halfgray}\chapterNumber\thechapter}}{0pt}%
    {\raggedright\spacedallcaps}[\normalsize\vspace*{.8\baselineskip}\titlerule]% 
}

but when I try to change color and size of the text, nothing happens.

 \documentclass[twoside,openright,titlepage,numbers=noenddot,headinclude, footinclude=true,cleardoublepage=empty,abstractoff, BCOR=5mm,paper=a4,fontsize=11pt,ngerman,american]{scrreprt} 

\input{classicthesis-config}
\usepackage{lipsum}

\begin{document}

\pagestyle{scrheadings}
\part{Introduction}
\chapter{First chapter}
\section{First section}
\lipsum[1-18]
\end{document}

Answer 1

The following example code shows where to insert the modifications required (the relevant lines of code are marked with % change color and size here). I used a shade of green and \huge, but of course you can use the attributes that best suit your needs:

 \documentclass[
   twoside,
   openright,
   titlepage,
   numbers=noenddot,
   headinclude, 
   footinclude=true,
   cleardoublepage=empty,
   abstractoff, 
   BCOR=5mm,
   paper=a4,
   fontsize=11pt,
   ngerman,
   american
   ]{scrreprt} 
\input{classicthesis-config}
\usepackage{lipsum}

\makeatletter
\ifthenelse{\boolean{@linedheaders}}%
    {% lines above and below, number right
    \titleformat{\chapter}[display]%             
        {\relax}
        {\raggedleft{\color{halfgray}\chapterNumber\thechapter} \\ }
        {0pt}%
        {\color{green!70!black}\huge\titlerule\vspace*{.9\baselineskip}\raggedright\spacedallcaps}% change color and size here 
        [\normalcolor\normalsize\vspace*{.8\baselineskip}\titlerule]%
    }
    {% something like Bringhurst  
    \titleformat{\chapter}[display]%
        {\relax}
        {\mbox{}\oldmarginpar{\vspace*{-3\baselineskip}\color{halfgray}\chapterNumber\thechapter}}
        {0pt}%
        {\color{green!70!black}\huge\raggedright\spacedallcaps}% change color and size here
        [\normalcolor\normalsize\vspace*{.8\baselineskip}\titlerule]% 
    }
\makeatother    
\begin{document}

\pagestyle{scrheadings}
\part{Introduction}
\chapter{First chapter}
\section{First section}
\lipsum[1-18]

\end{document}

The result:

Do not make the changes in the original .sty files (this is never recommended); do them in your .tex file surrounded with \makeatletter, \makeatother, as in my example code.