<div dir="ltr"><div><br></div><div>You should I 
think be able 
to add</div><div><br></div><div><br>\ifx\mathcolor\@undefined<br>  % simplified version of \mathcolor<br>  \protected\def\mathcolor{\begingroup\@ifnextchar[\mlx@mathcolora\mlx@mathcolorb}<br>  \long\def\mlx@mathcolora[#1]#2#3{\color[#1]{#2}#3\endgroup}<br>  \long\def\mlx@mathcolorb#1#2{\color{#1}#2\endgroup}<br>\fi</div><div><br></div><div>
to 
the end of miniltx.ltx  If 
that works for your real documents I'll arrange an update at

 ctan <br></div><div><br></div><div>David<br></div><div><br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Mon, 13 Jun 2022 at 10:56, Gérald Tenenbaum <<a href="mailto:gerald.ten@free.fr">gerald.ten@free.fr</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">

<div style="font-family:Lucida Sans"><div style="font-family:Lucida Sans">Hello,<br><br>Here is a small file 
that used to work but doesn't anymore:<br><br>\input eplain<br>\beginpackages<br>\usepackage{color}<br>\endpackages
 <br>\long\def\red#1{{\color{red}#1}}<br>\red{ab}<br>\bye<br><br><br>The
 error message is:<br><br>This is pdfTeX, Version 
3.141592653-2.6-1.40.24 (TeX Live 2022) (preloaded format=pdftex)<br> restricted
 \write18 enabled.<br>entering extended mode<br>(./essai.tex 
(/usr/local/texlive/2022/texmf-dist/tex/eplain/eplain.tex)<br>(/usr/local/texlive/2022/texmf-dist/tex/plain/graphics-pln/miniltx.tex)<br>(/usr/local/texlive/2022/texmf-dist/tex/latex/graphics/color.sty<br>(/usr/local/texlive/2022/texmf-dist/tex/latex/graphics-cfg/color.cfg)<br>(/usr/local/texlive/2022/texmf-dist/tex/latex/graphics-def/pdftex.def)<br>(/usr/local/texlive/2022/texmf-dist/tex/latex/graphics/mathcolor.ltx<br>/usr/local/texlive/2022/texmf-dist/tex/latex/graphics/mathcolor.ltx:21:
 Undefined control sequence.<br>l.21 \ExplSyntaxOn<br><br>Many thanks 
for help.<br>Regards,<br><br>Gérald<br></div></div>
</blockquote></div>