Ah, interesting!
It's not a bad idea, but one thing I'd advise you to bear in mind is that if you change certain colors in portraits, the game won't be able to palette swap them to match the character's army color--everyone's garments will show up purple in their portraits. Purples and reds in portraits and character sprites are therefore best left alone. (See: "Team Coloration" on or around page 85 of the manual for more details.)