I don't envy the pixel artists of old who had to make everything look as close to the same as they could on 27 different graphics and hardware standards that all had less than nothing in common and probably didn't even come from the same planet.
I've often wondered if artists of the era had access to some kind of palette conversion utility that would have automated some of the process involved with this. The basic shapes and outlines are in roughly the same place in each of the palettes, but some parts clearly had to be redrawn.
So much of this era and its tools are a complete mystery to me, and I wish we had more insight into what time saving measures developers took in order to streamline tedious tasks like this.
The art tool of choice for western gamedevs from the mid-80s through to mid-90s was Deluxe Paint by EA. Yes, that EA. It was originally designed for the Amiga, which had no end of palette bullshit to manage (whisper the words "extra half-brite" into a middle-aged european pixel artist's ear and watch them start having flashbacks), so there were probably processes... not that I'm too familiar with them.
Of note is that Dan Silva, the creator of Deluxe Paint, eventually left EA and moved over to another company where he was involved in the creation of a little piece of software called 3D Studio Max. It's not often that one guy's at the ground zero of two different revolutions in digital art...