i had an idea to have the whole thing be black and white but during the game, different, interchangable colours are in place of the black and white through post proccessing.

read through the rules and it isnt clear for me if this is allowed or not, it isnt using more than two colours at a given time and changing palletes is allowed.

Changing colours should be fine, as long as there are only 2 on the screen at a time