Coloring maths inside a tcolorbox

Question

My TeX level isn't the greatest, I am trying hard to improve but I am always struggling.

I would like to change the color of text inside a tcolorbox and that this color formatting survives a pagebreak. This doesn't seem to be possible according to what I found here : Maintaining text colour change in a breakable tcolorbox but it seems to be possible under XeTeX using fontspec. I have one last problem before I finally manage to solve this problem I have add for more than a year and many hours of swearing, this doesn't seem to apply to text put in math mode.

Here is a MWE based on what the link above :

\documentclass{article}
\usepackage{mathspec}
\setmainfont{Palatino}
\usepackage{blindtext}
\usepackage[breakable]{tcolorbox}

\begin{document}

    \begin{tcolorbox}[breakable]
      \blindtext[3]
      \addfontfeatures{Color=blue}
      \blindtext[3]
      \addfontfeatures{Color=red}
      \blindtext
      \addfontfeatures{Color=brown}
      \blindtext
      $$F=ma$$
      \blindtext[3]
      \addfontfeatures{Color=blue}
      \blindtext[3]
      \addfontfeatures{Color=red}
      \blindtext
      \addfontfeatures{Color=brown}
      \blindtext
    \end{tcolorbox}

\end{document}

Is it possible to get the $$F=ma$$ in brown ? Of course, any solution using the xcolor package won't work because of the incompatibility with tcolorbox, so I really need a font-associated solution. I had a look at the mathspec package but couldn't find the solution in the documentation. I somehow feel it shouldn't be that hard though.

Thanks in advance !

Answer 1

As it is rather improbable that there are page breaks in the math you could simply use \color:

\documentclass{article}
\usepackage{mathspec}
\setmainfont{TeX Gyre Pagella}
\usepackage{blindtext}
\usepackage[breakable]{tcolorbox}

\begin{document}

    \begin{tcolorbox}[breakable]
      \blindtext[3]
      \addfontfeatures{Color=blue}
      \blindtext[3]
      \addfontfeatures{Color=red}
      \blindtext
      \addfontfeatures{Color=brown}
      \blindtext\color{brown}
      \[F=ma\]
      \blindtext[3]
      \addfontfeatures{Color=blue}
      \blindtext[3]
      \addfontfeatures{Color=red}
      \blindtext
      \addfontfeatures{Color=brown}
      \blindtext
    \end{tcolorbox}

\end{document}

With lualatex instead of xelatex you could use the luacolor package. This will solve both problems:

\documentclass{article}
\usepackage{unicode-math}
\setmainfont{TeX Gyre Pagella}
\setmathfont{TeX Gyre Pagella Math}
\usepackage{blindtext}
\usepackage[breakable]{tcolorbox}
\usepackage{luacolor}

\begin{document}

    \begin{tcolorbox}[breakable]
      \blindtext[3]
      \color{blue}      
      \blindtext[3]
      \color{red}
      \blindtext
      \color{brown}
      \blindtext
      \[F=ma\]
      \blindtext[3]
      \color{blue}
      \blindtext[3]
      \color{red}
      \blindtext
      \color{brown}
      \blindtext
    \end{tcolorbox}

\end{document}

Answer 2

With XeLaTeX, you could patch the math commands (both display and inline) to include the color command. This ensures that the color is set on the page where the math starts.

This does not work with the TeX commands $ and $$ but only with the LaTeX equivalents \(\) and \[\]. It also does not work if your math crosses a page break (but of course it will work if a new math expression starts on the next page). If you want to use it with other math-like environments (equation, align, etc) then you should patch those separately.

Note that you should set \mycolor (or use \boxcolor) before you use math, otherwise you'll get an error. Also note that this takes effect outside of the tcolorbox as well, so you should probably put \def\mycolor{black} after each tcolorbox (or do that automatically with \AtEndEnvironment).

With a small macro you can set \color and \addfontfeatures with a single command.

MWE:

\documentclass{article}
\usepackage{mathspec}
\setmainfont{Linux Libertine O}
\usepackage{etoolbox}
\pretocmd{\[}{\color{\mycolor}}{}{}
\pretocmd{\(}{\color{\mycolor}}{}{}
\newcommand{\boxcolor}[1]{\def\mycolor{#1}\addfontfeatures{Color=\mycolor}}
\usepackage{blindtext}
\usepackage[breakable]{tcolorbox}

\begin{document}

    \begin{tcolorbox}[breakable]
      \blindtext[3]
      \boxcolor{red}
      \blindtext[3]
      \boxcolor{orange}
      \blindtext[3]
      \[F=m\times a\]
       \blindtext[3]
      \boxcolor{blue}
      \blindtext[3]
      \(1+1=2\)
      \boxcolor{red}
      \blindtext
      \boxcolor{brown}
      \blindtext
    \end{tcolorbox}

\end{document}

Result: