It's really fairly easy to do - I just guessed.
Just look at *\Beyond the Sword\Assets\XML\Interface\CIV4ColorVals.xml, work out how it works, make up your own colour, then change the Bannor entry in *\Fall from Heaven 2 030\Assets\XML\Interface\CIV4PlayerColorInfos.xml from UGLY_CYAN to NEW_COLOUR.
The Red/Green/Blue values are percentages, I think - not sure if I'm remembering that incorrectly.