New version of QuickMapServices is out. QuickMapServices is a QGIS plugin for painless adding basemaps as layers (more about the plugin). Update it in QGIS to have new version.
In the new version you can turn any added basemap into grayscale and also adjust contrast and brightness. To do it, simply open basemap properties and turn on Grayscale.
So, even if your favorite basemap doesn’t have grayscale version – you can have it.
OpenStreetMap Mapnik:
MapQuest OSM:
MapQuest Aerial:
The QuickMapServices plugin for QGIS v2.16 no longer offers Grayscale?
there was no special changes to the plugin for QGIS 2.16. If something doesn’t work consistently, please fill a ticket at https://github.com/nextgis/quickmapservices