\setuppagenumber[state=stop]

\pdfpxdimen49336sp

\startMPinitializations
    px:= .7528125;
    fooWidth  := 600px;
    fooHeight := 400px;
\stopMPinitializations

\startuniqueMPgraphic{foo}
    fill unitsquare xyscaled(fooWidth,fooHeight) withcolor transparent(1,0.2,blue);
\stopuniqueMPgraphic

\starttext
\framed[offset=overlay,width=600px,height=400px,framecolor=darkblue]{\uniqueMPgraphic{foo}}
\stoptext