horkrux: I've made a Cheat Engine table for Settler 3 that includes a script for changing your color to any of the 20 ones available. You can download it here

How to use:
1. Start up the game
2. Alt-Tab/Ctrl-Alt-Del out of the game (I think the former one is locked on the gog version)
2. Open the process "S3.exe" in CE
3. Open my table
4. Check below for "Change Colors", tick the box next to it to activate the script
5. Double-click in the "value" column next to "Player Color" and type in a value between 0-20. The default is 0, which corresponds to red
6. Go back to the game, start/load a map and enjoy the new color. Whoever had the color previously will be red now. This will persist until you change it again or deactivate the script

Changes will only take effect on each map load. If you want to change colors on the fly midgame to quickly test which colors the numbers correspond to, you can change the value of "PlayerColors" instead and type in the number directly. It's the very first number for player 1. You can also change the value for other players here, even have them all have the same color or other shenanigans.