[NTG-context] How to achieve transparency?

Hi, I wonder how transparency works in ConTeXt. I expect in the following example the text to be transparent. That's not the case. \definecolor [myI] [r=1, g=.9, b=.7, a=.8] \definecolor [myII] [r=0, g=.9, b=0, a=.8] \setupbodyfont [sans, 36pt] \def\t{Test} \starttext \bf \hbox{\color [myI] {\hbox to 0pt{\t}} \color [myII] {\t}} \stoptext Thanks in advance Marco