It's worth noting that, at least in Hercules Monochrome's case, different SCI drivers rendered colours differently. Back when I had my first computer that my dad had given me, a 286 with a 40MB HDD and a Hercules Monochrome card/monitor, I tried two different Hercules drivers from different SCI games and discovered this result. I don't know if the same is true for CGA or not. Wouldn't it be the driver files that do this though? Would hacking the driver files themselves provide any answers?
And yes, ScummVM has the same set of options for every game. That doesn't mean they all work for every supported engine. They're left over from when SCUMM was the only supported engine. Sometimes they implement them in other engines (sometimes in not the best ways), but mostly they don't really work in my experience.