First, you probably want to mask(componentMask) that scene texture to rgb only so it wont complain.
And to pass a random colour to the postprocess material I think that you can not use dinamic instances in postrpoces (don’t know why, maybe I’m wrong).
But what i’m sure you can do is to create an material collection parameter (in the same folder as the material for example) and in that MPC add a vector parameter and call it in the material.
Then in any bp (but most common in the player or level) create a mcp variable and asign the one u made. Then from that variable drag the set vector collection parameter node and select your vector. And as parameter just add a random vector