Alright, I actually managed to change the Julii faction to a T. Perfectly done.
All I did was set the green and blue values to 0 while the red values equal to the primary value.

However, I tried something different with the House of Brutii as another test. I switched all the colour values so they'd match their primary counterparts.

It came out as I wanted, however..their colour was a much, much brighter green. Heh.

I only tried that because I didn't know what else to do. I knew they weren't red, so I didn't know what colour to pick besides green. I then thought..I have no idea what to do with factions that don't use green/red/blue colours at all, so I was sort of confused and was hoping just doing that general formula as I did with the Brutii would solve any future woes.