\definecolor [trans] [a=multiply, t=.5, g=.5]

\startMPpage
    fill fullcircle scaled 12cm withcolor \MPcoloronly{trans};
    fill fullcircle scaled 10cm withcolor \MPcolor{trans};

    fill fullsquare xyscaled (15cm, 15cm)
      withshademethod "linear"
      withshadevector (0,1)
      withshadecolors (red,\MPcoloronly{trans})
      withtransparency \MPtransparency{trans}
    ;

\stopMPpage
