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