The MWE says it all:
% !TeX program = LuaLaTeX
% !TeX encoding = UTF-8
\documentclass{minimal}
\PassOptionsToPackage{monochrome}{xcolor}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\selectcolormodel{gray}
\filldraw[draw=red,fill=red] (1.5,4) rectangle (3.5,4.5);
\end{tikzpicture}
\end{document}
It draws the box. But the box is red. I expect it to be gray. Same with LuaLaTeX or pdfLaTeX compiler. TeXlive 2016 on Linux. Interface is TeXworks.
My first attempt with TikZ bombed. :(



xcolorplays almost almost no role in TikZ; I am not surprise if a command fromxcolordoes not work. (2) What exactly do you want? If you want a gray rectangle, you should tryfill=grayorfill=black!50. (3)\PassOptionsToPackage{gray}{xcolor}. – Symbol 1 Oct 23 '16 at 22:01\fill[red] (2,0) circle (1);, but\fill[fill= red] (2,0) circle (1);gives a red circle. It looks as if the key bypasses the color conversion. – Ulrike Fischer Oct 23 '16 at 22:27monochromeoption is not documented (by thexcolorpackage). – Paul Gaborit Oct 23 '16 at 23:14