Omnichild - Argh, yeah, it's frustrating with colors like that. I would say that the shirt is white, and the animators are just tinting it to make it look more interesting animated. They do that with Zoro's pants and bandana too - they're actually black, Oda colors them black, but they are tinted green in the anime. I've seen people try to do Zoro with dark green pants and bandana, but personally I was not a big fan of how it came out. As for Robin, if you went with black and white, I'm sure people who knew Robin would reconize it, and would look nicer than the weird lilac colored shirt. If you can find purple pants (try a thrift store!), awesome, but with the right hairstyle/wig, white and black could also look great!
As for Arabasta Nami's skirt…I do know where the white is coming from, it sometimes looks white in the earlier episodes (although that might have been my wash out eps I originally saw) as well as one statue that is probably bootleg I have. On official merchandice, like wall scrolls, pencil boards, and gashopons, it always looks light blue:
Oda's manga coloring (although done in pink) also leads me to believe its meant to be colored. However, again, I'd say its completely a matter of personal preference, and either way could be reconized.