Create a fog that desaturates the world

Im trying to create a particle system that adds ink splats to the scene around certain areas, but instead of appearing physically they just desaturate whatever theyre sitting over, creating blotches of black and white. But no matter what i try nothing seems to work at all, even masking out the transparency and then trying to lerp between saturated and desaturated using a post processing (Using the transparency as a mask).
Any ideas?

You can create a local PP material like this

But I can’t find a way to use that with a particle system…

Here, I’m setting the center of the PP using a MPC

Local PP MPC