How do I colour a specific part of render in compositor?

I feel like I should be able to work this out but I’ve hit a bit of a wall.

I have a night scene where I want to emulate the grain and desaturated look you get in low-light conditions, BUT, I want part of the image to retain the original colour and light reflections.

I rendered out various layers to exr with gloss/trans/emit etc components, but now I’m a bit stumped as to how to composit them.

Here is my wip image:

Here is approximately what I want (just quick lasso select in of doorway and creature, rest of image adjusted to b&w):

How do I do this? I tried layering various parts with math nodes but I always seemed to end up with everything black and white, or the image entirely lightened or darkened.

Probably lots of ways to solve this… Could try rendering just an emissive pass of the scene where you have an emissive white rectangle covering your doorway. Save that image and then use that as a mask for your compositing effects.

Good luck!