Created attachment 110059 [details] screenshot In Okular, when you open print menu, and you select "grayscale" option, Okular will still print using colours okular-17.08.1-1.fc27.x86_64
Created attachment 110060 [details] cups.log
This was caused by a Qt bug that has already been fixed: https://bugreports.qt.io/browse/QTBUG-54464. Once Fedora provides you Qt 5.11 (targeted for 3/15/2018: https://wiki.qt.io/Qt_5.11_Release) or backports the fix to Qt 5.9 or 5.10, then, the bug will be fixed for you.