% after a question on the metapost list eofill was added:

\starttext

\startMPpage

    fill
                fullcircle scaled 10cm --
                fullcircle scaled  8cm --
                fullcircle scaled  4cm -- cycle
        withcolor red/4
        withtransparency (1,.5) ;

    currentpicture := currentpicture shifted (-5cm,0) ;

    fill
                fullcircle scaled 10cm --
        reverse fullcircle scaled  8cm --
        reverse fullcircle scaled  4cm -- cycle
        withcolor green/4
        withtransparency (1,.5) ;

    currentpicture := currentpicture shifted (-5cm,0) ;

    eofill
                fullcircle scaled 10cm --
                fullcircle scaled  8cm --
                fullcircle scaled  4cm -- cycle
        withcolor blue/4
        withtransparency (1,.5) ;

    currentpicture := currentpicture shifted (-5cm,0) ;

\stopMPpage

\stoptext

