If I print a pdf file with Okular in grayscale (File -> Print -> Options -> Options -> Color Mode -> Grayscale), the file is printed in color. Other applications respect the grayscale setting in the printing dialog when printing the same file. The bug happens with Okular 1.2.3. Other printing settings within Okular seem to work though.
Bug 368758 describes the same issue and how things will change with the upcoming Qt 5.11 print dialog. *** This bug has been marked as a duplicate of bug 368758 ***