[NTG-context] Using transparency in METAPOST within textext()

Is it possible to get a transparent background in a textext()? The following minimal example doesn’t work. \starttext \setupMPpage [background=color, backgroundcolor=gray] \startMPpage draw textext("\definecolor[labelbackground][a=0.2,r=1,g=1,b=1]\framed[height=1.2em,background=color,backgroundcolor=labelbackground,foregroundcolor=black]{Text in Label}"); \stopMPpage \stoptext Thx. G