I think that Zoro would be white/black. That's Orzhov colours and they are all about protecting their select few. And as a bonus, Khans has a black and white Warriors theme.
Luffy would be red. Impulsive and passionate.
I like the idea behind Hakimi's Nami but as a land searcher she would almost definitely have to be green. Purely based on character I would go with blue and black. Maybe just black.
Chopper is green, maybe with white depending on abilties. Because animal and healer.
Robin: Seeking knowledge for the sake of knowledge. Definitely blue.
Brook: Black, I suppose. Purely because of the whole being dead thing. And I don't know what a musician would be…
Franky: As a ship builder he should be red with an artifact theme. Or possibly just an artifact.
Usopp: Green/blue works with his plants. Blue for ingenuity and green for growth.
Sanji: Blue for his cunning or red for his passionate side. I'm not super happy with this one because neither really captures him. Blue/red has a different flavour altogether. So I guess compromises have to be made.
If Sanji's mono red, Nami is mono black and Chopper is white/green then all five colours are represented twice and that's just pretty. Plus artifact Franky. It all works out~