How to render only one material in color, rest in grayscale?

I am working on a shooter FPS game, similar to Serious Sam or Painkiller. What I would like to achieve looks like this:

So basically, renders the whole image in grayscale, and renders in a specific color only materials which are marked by it.
How is this achievable? I’ve watched post-process tutorials, I can make the image grayscale, but how to filter a few materials so it renders them normally, just like in those 2 images?

I’m looking for a similar function but object based.
Basically have the post-process ignore certain objects.

Hopefully some wise and old UE wizard responds to this thread and hopefully the solution can be implemented in my scene as well!

You could use the Custom Depth buffer to filter what you want in colour. Take a look at some of the post process Outline tutorials on how to use the custom depth and simply use its value in a lerp between PostProcessInput0 and PostProcessInput0 > Desaturation.

Thanks Dec1234!
Just found one and it seems pretty straight forward.

I still don’t know how to do this. Can anyone else help me please?