When I place a grayscale jpg in a frame with a background colour of C100/Y85/M0/K38 and apply a colour of C0 / M84 / Y100 K13, it displays correctly, red image against blue background. When I export that to PDF, the PDF preview shows no red, and a black box with the grayscale image knocked out in blue. When I open that in Acrobat Pro, it again displays correctly.