[NTG-context] overprinting black

[Henning Hraban Ramm <texml@fiee.net>]

Hi, esp. Hans,

while finalizing the PDF for our latest book, I recognized a color problem:
The general black color is rendered not 100% black in CMYK mode, but in 
all channels (converted RGB, I guess).

With

\definecolor[cmykblack][k=1]
\setupcolors[textcolor=cmykblack]

this is ok, but the color mode is knockout instead of overprint (as it 
would be usual for CMYK black). That means, black text on a photo is 
white in the other plates.

If I define

\definecolor[cmykblack][k=1,a=multiply,t=1]
\setupcolors[textcolor=cmykblack]

it’s ok in some places, but it messes up many unrelated places, e.g. 
most images then also use this transparency mode.

\setupcolors[intent=overprint]
has no effect, as far as could find (while we have a warning in the 
wiki, that you should use this only for grayscale works).

What can I do?

Hraban
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