Question Shared Material and Shading

Hi

I have a scene in which all of my objects share the same material - their UVs are positioned on different colours on the image of the material.

During gameplay, I need some of the objects (at varying times) to become darker at stages until they become near-black.

I don't want to do a material swap, as with one material set up the way it is, it won't be efficient. I'm not experienced with shaders or vertex colouring and wondered if this is the path I should go down, or if anyone has any other ideas on how to tackle this problem, I'm all ears.

Thanks for any advice.