Question What is the general approach to have scene grayscale but some object excluded from the effect?

I would like the scene to turn grayscale except certain game objects such as characters and some UI.
I was testing with post processing effect but found that it seems only affect the whole screen.
I could use two cameras to limit the effect to non-UI but still not be able to preserve color for certain game object.

Any suggestion or idea would be appreciated.
Thanks.