I wanted to ask this question here, but I think I already found the solution.
Changing the Player's Kingdom colour in Diplomacy
Because TweakMB doesn't work for some weird reason?
Open
Factions.txt in your Diplomacy module directory.
Search for the line "0 fac_player_supporters_faction Player's_Supporters 0 16729139 "
The number "16729139" at the end of the line will be your Factions colour. This number is a Hexadecimal colour-code converted to a decimal number. So what you got to do, is find the colour you want in hexadecimal code. Most image-editing programs display Hex codes by default, but if you don't have one, check this website out:
http://www.2createawebsite.com/build/hex-colors.html
Once you picked a colour there, copy the Hex code at the bottom right and paste it into this converter:
http://www.easycalculation.com/hex-converter.php
For example, a dark blue Hex colour code: 0d3586
If I put it in the converter, it gives me this decimal code: 865670
I replace the pure red colour "16729139" in the Factions.txt with my dark blue colour "865670", then save the TXT and exit. Now when I start my own faction it will be blue. I read somewhere that if you already started your faction, you need to change your name (and back again) to make it pick the new colour.
I haven't tested this myself yet in Diplomacy, but it should work.