The color demo will show all colors on screen:
Color.zip 1.38K
132 downloads
I didn't see this file in the 7800 section of the site. It doesn't look like anything that somebody would claim copyright on but I apologize to the mods if I'm wrong about that.
A long time ago I made a palette in "The Gimp" using this demo, but it was based on an old version of MESS with a bad palette.
The most direct source of accurate colors would be to get the RGB values from
underball's palette he developed for the prosystem emulator. Not sure how that file is formatted though.