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I've been playing some games through ScummVM, and there's a cool feature that lets you load the game using whichever graphics mode the software originally supported. It also lets you use shaders to simulate a CRT, because these bare pixels were never meant to be seen with human eyes. I thought it was fun to compare the art from the different versions.
The posted image is from the EGA version
Here is the CGA:
And Here is Hercules(Amber):
CGA has some complexities. I haven't gone over everything, but I think the pictures are correct.
Check the wiki section on color palettes: https://en.m.wikipedia.org/wiki/Color_Graphics_Adapter
And as another poster said, VGA conversion is a problem all its own. See the 8-bit Guy's video: https://youtu.be/niKblgZupOc
VGA let's you pick a palette of 256 colours out of 262144, which is because the RGB components were actually on 6bit, not the full 8bit we would come to expect later. The 320x200 resolution would also be a sore point for artists and that's how you ended with quite a few games sticking to the EGA palette and using dithering to simulate more colours while using a superior resolution (640x480, or was it 800x600?). I've some vivid memories of Cobra Mission for example, or the Commander Keen saga.
Oh man, I remember that brief time where games had a "256 color mode."