I’m glad to hear that @8wai8 , thanks for following up. I marked your post as the solution for anyone finding this thread in the future.
I don’t know offhand how to make the display grayscale, but posting in the PureOS section was going to be my suggestion, glad you have already posted there 
(Grayscale on laptop and external display). KDE has some desktop effects that can desaturate background windows, but nothing for the whole display, and I don’t know about GNOME offhand.