how to color the header's bottom line or footer's upper line?

Question

The title is self-explanatory.

In my document's current state, I get this rendering result after compiling with XeLaTeX :

Here's a MWE of which packages and commands I use to obtain my current header style :

\documentclass{report}

\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage[utf8]{inputenc}

\usepackage{lipsum}% http://www.ctan.org/tex-archive/macros/latex/contrib/lipsum

\usepackage{titlesec}
\usepackage{xcolor}


\renewpagestyle{plain}[\small\sffamily\upshape\color{red}]{
    \headrule
    \sethead{\chaptertitle}{}{Université de la Rochelle}
}


\author{ME}
\title{Bash report}

\pagestyle{plain}
\begin{document}
\maketitle

\tableofcontents
\thispagestyle{empty}

\chapter{Matrices de convolution}
\lipsum

\lipsum
\end{document}

What I actually would like is to color "red" the lines created by the header and footer.

Answer 1

titlesec defines helper macros to set up the lines, this drops red into the head line

\documentclass{report}

\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage[utf8]{inputenc}

\usepackage{lipsum}% http://www.ctan.org/tex-archive/macros/latex/contrib/lipsum

\usepackage{titlesec}
\usepackage{xcolor}

\makeatletter

\newcommand\setheadrule[1]{%
  \ifdim#1=\z@
    \let\makeheadrule\@empty
  \else
    \def\makeheadrule{{\color{red}\rule[-.3\baselineskip]{\linewidth}{#1}}}%
  \fi}

\makeatother


\renewpagestyle{plain}[\small\sffamily\upshape]{
    \headrule
    \sethead{\chaptertitle}{}{Universite de la Rochelle}
}


\author{ME}
\title{Bash report}

\pagestyle{plain}
\begin{document}
\maketitle

\tableofcontents
\thispagestyle{empty}

\chapter{Matrices de convolution}
\lipsum

\lipsum
\end{document}

The foot rule command is by default

\newcommand\setfootrule[1]{%
  \ifdim#1=\z@
    \let\makefootrule\@empty
  \else
    \def\makefootrule{\rule[.7\baselineskip]{\linewidth}{#1}}%
  \fi}

But not apparently used in this MWE so I didn't change it/

Answer 2

I solved the problem by slightly modifying David's code:

\makeatletter
\renewcommand\setheadrule[1]{%
  \ifdim#1=\z@
    \let\makeheadrule\@empty
  \else
    \def\makeheadrule{{\color{red}\rule[-.3\baselineskip]{\linewidth}{#1}}}%
  \fi}
\makeatother

Using David's code, I had the same error message reported by vdegenne in his first comment (solved by him by adding the pagestyles option to the titlesec package as said in his second comment). Since their solution did not work for me, I worked out my own by just substituting \renewcommand instead of \newcommand in Dave's code: this simply redefines the command avoiding any "already defined!" run time error, and produces the sought for graphical output.