Nermander
The choice is made when a selected spot color is applied to an object. I am not sure of all the background issues with color programming, but the pdf file type already accepts this within its formatting otherwise Quark, Corel, Photoshop, Illustrator, InDesign would not be able to do it... Most of these applications can even apply spot colors to Greyscale images, not just single bit b/w images. I deal with several graphic designers on a monthly basis, and I would say at least 20%+ of the files that I need to print have spot colors applied to bitmaps, but I live in the world of spot color printing.
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Utnik
Thanks for the trick with photoshop-->pdf placement, I will have to give that a try sometime in the next week or so, but my ultimate goal is to use only FOSS software.