Change color system for part of a document?

Is there a way to change the color model to monochrome for a section of a document? I would like to make a section mononcrome without affecting the headers. Is this possible? I know about \usepackage[mononcrome]{xcolor} and \passOptionToPackage[mononcrome]{xcolor} but these can only be applied in the preamble, and I think they affect the headers as well?

MWE below

\documentclass{article}
\usepackage{lipsum,fancyhdr,xcolor}

\pagestyle{fancy}

\begin{document}

\section{Colors}

\lipsum[1-2]

\section{Monochrome}

\lipsum[3]\textcolor{red}{This should become gray}
\lipsum[4]\textcolor{green}{this too}
\lipsum[5]\textcolor{blue}{and this}

\section{Colors}

\lipsum[5]\textcolor{red}{This should stay red}
\lipsum[66]\textcolor{orange}{same with this}
\lipsum[75]
\end{document}

• Note that monochrome does not convert colors into grayscale, but instead all colors are turned into black. For grayscale you can use the gray package option - however that does not seem so easy to switch as the monochrome setting as demonstrated in the answer below. Oct 2 '19 at 20:13

Here, I looked in the xcolor package and found what was executed by the monochrome option. I then just placed it directly in the document in a group-limited way.

EDITED to make it into the monochrome environment.

\documentclass{article}
\usepackage{lipsum,fancyhdr,xcolor}

\pagestyle{fancy}

\makeatletter
\newenvironment{monochrome}
{\begingroup
\colors@false
\def\set@color{\c@lor@special\m@ne
{color push \current@color}\aftergroup\reset@color}%
\def\reset@color{\c@lor@special\m@ne{color pop}}%
\def\set@page@color{\c@lor@special\m@ne{background \current@color}}
}
{\endgroup}
\makeatother

\begin{document}

\section{Colors}

\lipsum[1-2]

\section{Monochrome}

\begin{monochrome}
\lipsum[3]\textcolor{red}{This should become gray}
\lipsum[4]\textcolor{green}{this too}
\lipsum[5]\textcolor{blue}{and this}
\end{monochrome}

\section{Colors}

\lipsum[5]\textcolor{red}{This should stay red}
\lipsum[66]\textcolor{orange}{same with this}
\lipsum[75]
\end{document}


• Yes, I know that it would seem the monochrome portion has no grayscale, but is merely black. But that still may be useful to the OP. Oct 2 '19 at 20:44
• I agree, and the OP also asked for the monochrome option specifically, that's why I commented on his question and not on your answer (which is very nice, +1). Oct 2 '19 at 20:53
• Not really suited to write a book on QCD, but who cares? ;-)
– user194703
Oct 3 '19 at 3:40
• Hi, is it possible to make everything grayscale instead of monochrome? I thought monochrome was grayscale, because of my poor english =( Oct 3 '19 at 8:56

To help with the discussion under Steven B. Segletes's answer, and since I can't comment there, here's how to set grayscale within a group \selectcolormodel{gray}, or as an environment:

\newenvironment{grayscale}
{\begingroup \selectcolormodel{gray}}
{\endgroup}


This might answer the intended question in a straightforward manner.

Any color model could be chosen instead of gray, but it's not obvious why that'd be desired.