Color Underline Section Headings

Question

I would like to underline section headings using different colors (e.g. red for section, yellow for subsection). So far I have only achieved to change the text color of the headings (see code), but I want to underline the headings and then change the line color while keeping the text color black.

\documentclass[a4paper,12pt]{scrartcl}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[colorlinks=true]{hyperref}

\addtokomafont{section}{\color{red}}
\addtokomafont{subsection}{\color{yellow}}

\begin{document}
\section{Section}
\subsection{Subsection}
\end{document}

PS: I don't know why the code example needs \usepackage[colorlinks=true]{hyperref}, but it doesn't compile without.

Answer 1

Update

Since KOMA-Script version 3.19 command \sectionlinesformat can be redefined to get the desired result using package ulem:

\documentclass[a4paper,12pt]{scrartcl}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{xcolor}
\usepackage[normalem]{ulem}
\usepackage{blindtext}

\makeatletter
\renewcommand{\sectionlinesformat}[4]{%
  \ifstr{#1}{section}
    {\colorlet{ulinecolor}{red}}
    {\ifstr{#1}{subsection}{\colorlet{ulinecolor}{green}}
      {\colorlet{ulinecolor}{.}}}%
  \@hangfrom{\hskip #2\expandafter\sectionuline\expandafter{#3}}{\expandafter\sectionuline\expandafter{#4}}%
}
\makeatother

\newcommand\sectionuline{%
  \bgroup\markoverwith{\textcolor{ulinecolor}{\rule[-0.75ex]{2pt}{0.4pt}}}%
  \ULon%
}
\colorlet{ulinecolor}{black}

\addtokomafont{section}{\Huge}

\begin{document}
\section{Really long section title that needs more than one line}
\subsection{Subsection}
\subsubsection{Subsubsection}
\end{document}

results in

---

Original answer

If you want to color only the lines you can load package ulem and define new commands for colored lines as suggested in its manual:

\newcommand\sectionuline{%
  \bgroup\markoverwith{\textcolor{red}{\rule[-0.5ex]{2pt}{0.4pt}}}%
  \ULon%
}
\newcommand\subsectionuline{%
  \bgroup\markoverwith{\textcolor{blue}{\rule[-0.5ex]{2pt}{0.4pt}}}%
  \ULon%
}

Then you can add these commands to the font settings of section and subsection by

\addtokomafont{section}{\sectionuline}
\addtokomafont{subsection}{\subsectionuline}

Warning: \...uline must be the last added one to the font settings because of they take an argument. Additionally this \...uline commands are formating commands so this is something like a misuse of the font settings.

Code:

\documentclass[a4paper,12pt]{scrartcl}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{xcolor}
\usepackage{ulem}

\newcommand\sectionuline{%
  \bgroup\markoverwith{\textcolor{red}{\rule[-0.5ex]{2pt}{0.4pt}}}%
  \ULon%
}
\newcommand\subsectionuline{%
  \bgroup\markoverwith{\textcolor{blue}{\rule[-0.5ex]{2pt}{0.4pt}}}%
  \ULon%
}

\addtokomafont{section}{\sectionuline}
\addtokomafont{subsection}{\subsectionuline}

\begin{document}
\section{Section}
\subsection{Subsection}
\end{document}

Answer 2

Thanks for all contributions. for a while I’m looking for the simplest (without using koma ) way to underline section and subsection without affecting chapter heading, I find several talks about that but I got always troubles and affecting chapter heading. Finally it works for me , hope this helps others ….

\usepackage{titlesec}
\usepackage{xcolor}
\usepackage[normalem]{ulem}

\titleformat{\chapter}[display]
{\normalfont\huge\bfseries\centering\color{blue}}{\sc{\chaptertitlename\ \thechapter}}{20pt}{\Huge}

\titlespacing*{\chapter}
    {15pt}
    {15pt}
    {50pt}
\titleformat{\section} {\normalfont\Large\bfseries\raggedright}{\thesection}{1em}{\uline}

\titleformat{\subsection}{\normalfont\large\bfseries\raggedright}{\thesubsection}{1em}{\uline}