Hello,
Everything is in the title, as this example shows.
Thanks,

Fabrice

####################################################################
\usemodule[tikz]
\usetikzlibrary[matrix]

\setupbodyfont[pagella,11pt]

\starttext

\starttikzpicture
  \matrix [matrix of math nodes,ampersand replacement=\&]
  {
    a_8 \& a_1 \& a_6         \\
    a_3 \& a_5 \& |[red]| a_7 \\
    a_4 \& a_9 \&     a_2     \\
  };
\stoptikzpicture

\stoptext

####################################################################
tex error       > tex error on line 14 in file /home/fab/ntg49.tex: ! Argument of \tikz@lib@matrix@with@options has an extra }

<inserted text>
\par
<to be read again>
}
l.14   }
      ;

 4     \setupbodyfont[pagella,11pt]
 5    
 6     \starttext
 7    
 8     \starttikzpicture
 9       \matrix [matrix of math nodes,ampersand replacement=\&]
10       {
11         a_8 \& a_1 \& a_6         \\
12         a_3 \& a_5 \& |[red]| a_7 \\
13         a_4 \& a_9 \&     a_2     \\
14 >>    };
15     \stoptikzpicture
16    
17     \stoptext