Add a button to the symbology pane to flip the entire symbology of a layer to greyscale and back.
When you have a complex, pretty, full colour symbol set set up and you hear the client may want to print it in black&white only it will be handy to have a feature to flip the entire style to greyscale so you can see what it looks like and make adjustments to the colours.
This should not be a one-way conversion but a just a state from which you can revert back to the full colour version.
This should not be an output stage (pdf, etc) setting but layer specific as it is handy to set a map as greyscale and use colour on a single other layer for dazzling effect.