At the moment when a point cloud is attached to a file it isn't possible to print to monochrome in the original or in any files that it is referenced into. To get it to work you have to remove the point cloud from the file. Then you can change printing to monochrome. If it is a reference file you can also 'merge to master' to print monochrome, but then you loose the advantages of dynamic references. It would be great to have the option to print to monochrome even when a point cloud is loaded.