% This will work with mplib of December 2012 (prescriptpart / postscriptpart + a fix).

\starttext

\startMPpage
    draw image (
        draw fullcircle scaled 20cm withpen pencircle scaled 20mm withcolor red   withtransparency (1,.40) ;
        draw fullcircle scaled 15cm withpen pencircle scaled 15mm withcolor green withtransparency (1,.30) ;
        draw fullcircle scaled 10cm withpen pencircle scaled 10mm withcolor blue  withtransparency (1,.20) ;
    )
        withcolor blue
        withtransparency (1,.125) % selectively applied
        withpen pencircle scaled 10mm
    ;
    setbounds currentpicture to fullsquare scaled 25cm ;
\stopMPpage

\startMPpage
    draw undecorated (
        draw fullcircle scaled 20cm withpen pencircle scaled 20mm withcolor red   withtransparency (1,.40) ;
        draw fullcircle scaled 15cm withpen pencircle scaled 15mm withcolor green withtransparency (1,.30) ;
        draw fullcircle scaled 10cm withpen pencircle scaled 10mm withcolor blue  withtransparency (1,.20) ;
    )
        withcolor blue
        withtransparency (1,.125) % selectively applied
        withpen pencircle scaled 10mm
    ;
    setbounds currentpicture to fullsquare scaled 25cm ;
\stopMPpage

\startMPpage
    draw decorated (
        draw fullcircle scaled 20cm withpen pencircle scaled 20mm withcolor red   withtransparency (1,.40) ;
        draw fullcircle scaled 15cm withpen pencircle scaled 15mm withcolor green withtransparency (1,.30) ;
        draw fullcircle scaled 10cm withpen pencircle scaled 10mm withcolor blue  withtransparency (1,.20) ;
    )
        withcolor blue
        withtransparency (1,.125) % selectively applied
        withpen pencircle scaled 10mm
    ;
    setbounds currentpicture to fullsquare scaled 25cm ;
\stopMPpage

\startMPpage
    draw redecorated (
        draw fullcircle scaled 20cm withpen pencircle scaled 20mm withcolor red   withtransparency (1,.40) ;
        draw fullcircle scaled 15cm withpen pencircle scaled 15mm withcolor green withtransparency (1,.30) ;
        draw fullcircle scaled 10cm withpen pencircle scaled 10mm withcolor blue  withtransparency (1,.20) ;
    )
        withcolor blue
        withtransparency (1,.125) % selectively applied
        withpen pencircle scaled 10mm
    ;
    setbounds currentpicture to fullsquare scaled 25cm ;
\stopMPpage

\stoptext
