Colors changing when applying "transparent"

[Willi Egger <w.egger@boede.nl>]

Hi,

I would like to use transparnt colors in one of the documents. However the 
resulting color is different with transparent color. Could anyone try the 
following code twice setting the %-sign in front of the fill-statement:

\setupoutput[pdftex]
\starttext
\setupcolors[state=start]

\startuseMPgraphic{tBG}
         path p ;
         p := unitsquare xscaled \overlaywidth yscaled \overlayheight ;
         fill p withcolor \MPcolor{white}; % With normal colors, everything 
is fine (blue is really RGB(0,0,1)
         % fill p withcolor transparent("normal",.5,\MPcolor{white}); % 
This however results in fading of ALL of the colors...
\stopuseMPgraphic

\defineoverlay[textBG][\useMPgraphic{tBG}]

\setupbackgrounds[page][background=color,backgroundcolor=blue]
\setupbackgrounds[text][background=textBG]

\input tufte
\stoptext

I would expect that the optical appearance of blue shoud not change after 
application of the transparent color to the gray areq.

Regards Willi