[NTG-context] transparent color

I would like to use a transparent version of an existing color. Thus \definecolor[transparentcolor][color,t=0.2,a=1] instead of \definecolor[transparentcolor][r=..,g=..,b=..,t=0.2,a=1] because I do not know the rgb values of the color that must be made transparent, only its name. How to accomplish? yours sincerely dr. Hans van der Meer