Possible to have certain objects in the scene cast shadows that have a different colour?

I'd like to have certain objects cast shadows of a particular 'special' colour, so that I can apply a post-processing effect to them. Or perhaps have an additional shadow map that would only include specific objects in it, that I can check for in a post-processing shader. Is this possible? From what I've read it looks like I'm going to have to dig into SRP and write some custom scripts. Any pointers or suggestions would also be appreciated, thanks!