Macro for removing font changes that returns number in default font in KaTeX

Question

So this question explains why \mathnormal doesn't actually return numbers in the default font, but doesn't offer any solution for achieving this.

I'm working with KaTeX and have a set of macros for bolding & colouring symbols, but would like to be able to define an \unstyle macro that exempts part of an expression. For example, if \blue{} applies \boldsymbol and {\color{blue}} to its argument, I'd like to be able to write something like:

\blue{x^3 + 2x \unstyle{+ x} - 12}

such that everything except the "+ x" is in boldsymbol and colour{blue} and the "+ x" is in the default ("normal" math).

NB. double invocation of \blue, like:

\blue{x^3 + 2x} + x \blue{- 12}

does not solve the issue, as it breaks the rendering of + and - as operators:

Any suggestions as to how to achieve this? I've played around with every combination of the fonts options available on KaTeX; however, the ones that resolve the issue for numbers change the font for the letters and vice-versa. I'm surprised that there isn't an actual "normalmath" (i.e. defaultmath) option.

Any advice would be greatly appreciated.

Answer 1

\newcommand\unstyle[1]{\textcolor{normalcolor}{#1}}

$\blue{x^3 + 2x \unstyle{+ x} - 12}$

$x^3+2x+x-12$

Of course, this doesn't work in standard LaTeX, but it could be adjusted.

\documentclass{article}
\usepackage{xcolor}

\newcommand{\blue}[1]{\textcolor{blue}{#1}}
\newcommand{\unstyle}[1]{\begingroup\normalcolor#1\endgroup}

\begin{document}

$\blue{x^3 + 2x \unstyle{+ x} - 12}$

$x^3+2x+x-12$

\end{document}