I think I now have all the steps I need. Thank you everybody.
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:
* Please note, not recommended for prepress!
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
- 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!
* Please note, not recommended for prepress!