documentclass[UTF8]{article}

begin{document}
    section{section I}
        subsection{subsection I}
    section{section II}
        subsection{subsection II}
end{document}

documentclass[utf8]{article}
usepackage{titlesec,xcolor}

titleformat{section}{bfseriesLarge}{textcolor{orange}{rule[-3pt]{5pt}{1em}}}{5pt}{}

begin{document}
    section{section I}
        subsection{subsection I}
    section{section II}
        subsection{subsection II}
end{document}