one bit transparency

[bitrat]

I think I now have all the steps I need.  Thank you everybody.  

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Somewhat away from my original question and not my specific goal, but key to it if anyone else is interested...

To simulate CMYK process* using GIMP and Inkscape:

In GIMP:

• decompose RGB image to CMYK layers
  
• create image mask for each layer and initialise with Grayscale copy of layer
  
• convert layers back to RGB
  
• flood fill each layer with its process colour (select cmyk with the palette selection buttons above sliders)
  
• turn off visibility for other colours and export a png for each layer
  

In Inkscape:

• import the png's
  
• create separate layers for each process colour
  
• set each layer's Blend mode to Darken or Multiply
  
• move each png to it's corresponding layer
  
• align the images
  
• ...and viola!
  

Finally, to disable antialiasing for pixel level control, set Image Rendering to crisp-edges in Object properties.

* Please note, not recommended for prepress!