I wrote a devlog on how I did the black and white shader in Godot: https://rudyvic.itch.io/light-age/devlog/242548/light-age-black-and-white-shader
It's really straightforward, but for me, this was a big problem because I didn't have any type of experience with shaders (this was my first time using them, and I have understood that they are really powerful). For this reason, the first thing that I did for the game was this shader, and I have started to do the game only after it worked properly.