\starttext

\def\foo{\frac{\frac{1}{\frac{1}{x}}}{\frac{1}{\frac{1}{x}}}}

\setupmathfence[brace][color=darkyellow]
\setupmathfence[bar][color=darkred]
\setupmathfence[doublebar][color=darkred]

$\left\|     \foo \right\|$
$\left|      \foo \right|$
$\left{      \foo \right}$
$\lfence{||} \foo \rfence{||}$

$\fenced[doublebar]{\foo}$

\definemathfence [db] [bracket] [command=yes,color=darkblue]

$\fenced[db]{\foo}$

\definemathfence [db] [doublebar] [command=yes,color=darkgreen]

$\fenced[db]{\foo}$

\stoptext

