\setupcolors[state=start]
\definecolor[ghost] [a=1,t=.7,s=1]

\starttext
\startMPcode
  fill unitsquare xyscaled(3cm,3cm) withcolor red;
  fill unitsquare xyscaled(1cm,1cm) withcolor \MPcolor{ghost}; % broken
\stopMPcode

\framed[background=color,backgroundcolor=red,foregroundcolor=ghost]{A transparency test} % OK
\stoptext