pandoc -f latex -t native
\newcommand{\mycolor}{red}

\includegraphics[width=17cm]{\mycolor /header}
Magnificent \mycolor{} header.
^D
[Para [Image ("",[],[("width","17cm")]) [Str "image"] ("red/header",""),SoftBreak,Str "Magnificent",Space,Str "red",Space,Str "header."]]