The pallete shifting was implemented in the game. At the time, the video cards worked by having a 4 or 8 bit image resident in video RAM and also a 16 or 256 e

The pallete shifting was implemented in the game. At the time, the video cards worked by having a 4 or 8 bit image resident in video RAM and also a 16 or 256 entry pallete of colors as well (18 bit was common in VGA, you didn't even get 24bit palletes!). The card would do the pallete transform every frame pixel by pixel as it scanned out the image to the CRT. The point being: at no point in time did a 18 bit, full color image exist anywhere in RAM. That meant even on a machine that was too slow to even just upload a whole frame of 320x200 8 bit pixels 30 times/second, you could still easily upload a pallete of 256 colors every frame. Just doing that would change how the whole image displayed without actually doing the work of touching any of the pixels.