Setting a greyscale (or monochrome) color scheme

Solution 1:

Compiz has a few filters which supposedly can do what you want.

1. Install Compiz Config Setting Manager from Ubuntu Software Center or with sudo apt install compizconfig-settings-manager.
2. Run it and under Accessibility category you'll find Color Filter and Opacity, Brightness and Saturation filters.
3. There you'll be able to enable them, assign a keyboard shortcut and edit the filters' settings.

Now, I have to admit that I wasn't able to test it since I only have Ubuntu in a virtual box without 3D acceleration, but hopfully it'll give you some pointers.

Also, please don't get too excited modifying Compiz settings - it is quite easy to break Unity by doing so. The filters I mentioned above should be quite safe though.