If you visualise the depth buffer of a game (stay with me), you can make it look like the game is running in black and white, a la Limbo.
It isn't a very practical thing to do, but it does look very nice, as kidaXV shows/explains here.
Here's Borderlands 2:
Here's Fallout 4:
And here, most fittingly, is Dark Souls:
This isn't a recommended way to actually play through games, since there are times -- menus especially, since there's no depth there to visualise -- when the effect will be a pain in the arse. Plus, there is no HUD, so again, that's a problem.
But for making pretty videos, it's very useful.