How to convert image to CMYK in GIMP?
I realize this is an old question, but I just stumbled upon it and found the answer. After you separate the image into the CMYK channels, you get a .tif image with 4 layers (one for each color). This image looks inverted because each area where this is white/grey represents a certain amount of cyan, magenta, yellow, or black.
Once you have this, just go to Image -> Separate -> Export and export your image as a .tif. This will repackage your image with the CMYK profile you choose, and when you view it, it will be the right print colors.
You can also use Image -> Separate -> Proof, and select the color profile of your monitor and it will generate a .tif preview with all the right colors. It's a pretty nice plugin!
Super easy with ImageMagick (preinstalled in every Ubuntu):
convert input.jpg -colorspace cmyk -compress LZW output.tif
Note: the exported file would be quite large (no matter the tool), so it's better to include compression - for TIFF the most widely used is LZW.
More info e.g. here.
Have you tried this:
Separating an image:
To convert an RGB image to CMYK format, bring up the right-button menu, and go to "Image->" If the plugin in installed correctly, there will be a new menu, "Separate". From this new menu, select "Separate (normal)"; you will be prompted to select an RGB source profile, and a CMYK destination profile. If you have installed the Adobe and sRGB profiles as per the instructions in the archive, you can just accept the defaults for testing, otherwise you'll have to locate the profiles manually.
A new image will be created with four greyscale layers, named "C", "M", "Y", and "K".
If you have loads of memory to spare, you can use the "Separate (colour)" option; this will perform the same operation, but the new image will contain five layers: The first, "Background" will be white, and the other four will be solid Cyan, Magenta, Yellow and Black, with the separated image data in layer masks. In addition, the layer modes for the four colours will be set to "Darken Only". This gives a rough reconstruction of the colours, and is the next best thing to a true CMYK painting mode, since you can paint on the layer masks, and see the results in realtime.
NEW for 0.3 - the "primary" colours chosen for the "Separate (colour)" mode are now much more akin to the primaries used in printing, which are nowhere near as bright, saturated and downright lurid as pure screen Cyan and Magenta! This gives a far more realistic preview of the colours.
source : here