% \definecolor[red]  [r=1,t=.5,a=1]
% \definecolor[green][g=1,t=.5,a=1]
% \definecolor[blue] [b=1,t=.5,a=1]

\startMPpage
    fill fullcircle scaled 9cm shifted (0cm,0cm) withcolor .5 * namedcolor("red") ;
    fill fullcircle scaled 9cm shifted (3cm,0cm) withcolor .5 * namedcolor("green");
    fill fullcircle scaled 9cm shifted (6cm,0cm) withcolor .5 * namedcolor("blue") ;
\stopMPpage

