Color symbols individually in math mode (unicode-math clash)

Question

The solution in the post https://tex.stackexchange.com/a/100625 is highly efficient. Unfortunately it can not be used with unicode-math, which erases the individualized symbol colors. While unicode-math does allow individual symbol coloring via \setmathfont, if too many symbols are independently colored in this way, the source will not compile.

Is there any way to make the \colorizemath code (linked above) compatible with unicode-math ?

Answer 1

The following snippet causes the variable x to display blue in math mode without affecting its appearance in text mode. Further, it seems to get along well with unicode-math. (This snippet can be extended to independently colour many symbols, and it compiles rapidly.)

\begingroup
\lccode`~=`x
\lowercase{\endgroup
    \def~{{\color{blue}\symnormal{x}}}%
 }%
\mathcode`x="8000

(Code modified from https://texfaq.org/FAQ-activechars)

Answer 2

The lcc lower-case-code method is a lookup.

An expl3 lookup method could be used.

Here, a token-list mapping function as proof-of-concept:

A key-value property-list could also be used in expl3, as an alternative.

Much patience needed, to set up every symbol (and operator).

MWE

\documentclass{article}
\usepackage{xcolor}
\usepackage{unicode-math}
\setmathfont{TexGyrePagella-Math}
\usepackage{xparse}


\ExplSyntaxOn

\cs_new:Npn \sym_to_col:n #1
{
    \tl_set:Nn \l_tmpb_tl { #1 }
        \str_case:VnTF 
            \l_tmpb_tl      
            {
                { 𝒚 } { {\color{red} 𝒚 } }
                { + } { {\color{green} + } }
                { m } { { {\colorbox{blue}{\color{yellow} $m$} } } }
                { n } { {\color{blue} n } }
                { ( } { {\color{brown} ( } }
                { ) } { {\color{brown} ) } }
                { 𝒯 } { {\color{violet} 𝒯 } }
            }
                 { }  % true
                    { #1 } % false
}


\NewDocumentCommand \colourmaths { m } {
    \tl_set:Nn \l_tmpa_tl { #1 }
    \if_mode_math:
        \tl_map_function:NN 
                \l_tmpa_tl 
                \sym_to_col:n 
    \else:
            \tl_use:N \l_tmpa_tl
    \fi:

}

\prop_gset_from_keyval:Nn \g_tmpa_prop
{
𝒚=blue,
}

\ExplSyntaxOff



\begin{document}

Text mode:

abc \colourmaths{mno} 

\bigskip
Math mode:

$xyz = \colourmaths{𝒚mn+qrs(0)𝒯}$

\end{document}