As I understand, every single pixel in your game must be a color from the palette. Anti-aliasing, shading and post processing are all forbidden since they produce pixels of different colors.
e.g. say you’re only allowed to use black and white but you use anti-aliasing
see how anti-aliasing has created extra gray pixels that would not be allowed? The same goes for any other thing that creates pixels of different colors from the ones in the palette.