Well, It could just be a plastic film or something. I'm not sure where I got the idea, someone might have suggested it to me so I parroted it. It seemed plausible because it has to be outside of the emulation. Something as vital as palette settings should be emulated along with the game, I think, especially given the dreadful appearance of Contra. People do bother to make screen borders, so why not files for proper palette settings?
I can't find much info about Jamma color calibration. Shouldn't the settings of the 'palette dials' be in the service manual? They usually have instructions for the hardware switches and stuff.