People have figured out how to hack them in the roms for certain games like CPS2 fighters and Mortal Kombat and King of Fighters games if you Google. There isn't going to be a universal way to hack hex to do this across all games, and in fact even within a system like CPS2 I think the method you'll find on the web for them only works for Capcom fighters on it because they reused code. I've never heard of a way to change pallets within mame. Maybe it was added to one driver once, I don't remember... Cheats like that tend to only get added in the source if they have some benefit to developers. It might be possible to add stuff like that as an external cheat code though but it would still be game specific and I doubt you could get someone to add it for you because color changes generally aren't interesting to coders.
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