Is there a way to ensure that only a small area of the level is visible in black and white?
Visible even from afar without necessarily entering that area as in the photo.
Or create a sort of window from which you can see everything in black and white
I think where this piccy comes from describes how to do it, no?:
I’m sure it’s very well explained, but I can’t get it to work. Could you help me? (sorry for my English)
So, in the blog, they’re showing how to make something that is between 0 and 1, depending on world location. We all know what to do with something that’s between 0 and 1, right? Stick it into a lerp 
:
It’s not stunningly obvious, but the center cube is in b&w…
Thank you very much and good luck with your projects!