Using the standard Red/Yellow/Blue colour wheel you get something like this:
This almost exactly matches the straw hats, with only Chopper changing from magenta to pink (likely cause it just suits Chopper better and is more of a prominent colour:
Before someone says that colours aren't at all important, here's a tweet from Greg:
You can even check and see that after FMI arc, every single kimono Jinbe wore was in the colour ochre showing his efforts to associate him with the colour (outside the manga of course). It's certainly something Oda cares about and likes to take into consideration. With that in mind, the missing colours from the wheel are violet, vermillion and lime.
Fortunately the last popularity poll displayed the top 30 characters in what appears to be their official colours (matches up with every known official colour).
Remember what those missing colours were...violet, vermillion and lime... Look which characters just happen to be sitting right next to each other in those very same missing colours.
Problem solved. Oda used the colour wheel for the 11 straw hats and Luffy's two brothers.