Vous pouvez utiliser la commande `\selectcolormodel{gray}`
\documentclass{scrartcl}
\usepackage{fontspec}
\usepackage[usenames,dvipsnames]{xcolor}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\selectcolormodel{gray}% pour un effet local
\draw [Red] (0,0) -- (3,0) node [midway, below] {Du texte};
\end{tikzpicture}
\end{document}
Une autre méthode
\documentclass{scrartcl}
\usepackage{fontspec}
\usepackage[monochrome,usenames,dvipsnames]{xcolor}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[every node/.style={black}]
\draw [BrickRed] (0,0) -- (3,0) node [midway, below] {Du texte};
\end{tikzpicture}
\end{document}
