It really bothers me how in the anime they 'whitewashed' Robin and Usopp after the timeskip. You can really see how light their skin complexion are in comparition to before, especially since Usopp is canoncally african.
I’ve written about this before (here and here), and I think the problem rests more with Oda than Toei (for once). I think it is a shame that Toei lightened Usopp and Robin’s skin colors after the time skip, but Oda’s the one who has always drawn them as light skinned in the first place. Miss Monday is the only character that Oda has ever colored with dark skin. Other characters we think of as dark skinned, Oda always draws light skinned: Daz Bones, Usopp, Aokiji, Blackbeard, Wiper—they’re all light skinned the way Oda colors them, and it was only the anime that ever made them darker skinned. Here are various characters typically thought of as dark skinned that Oda all draws otherwise
Even Mr. 5 was colored with skin just as light at Mr. 4′s by Oda
To this day, the only character Oda has colored with dark skin is Miss Monday
So yeah. Oda himself has done very little to give his cast diverse skin colors. And like I said with the thing about Usopp being said to be African (if he were real), it’s an issue because he’s the only one of the Straw Hats who was placed as being from an ENTIRE CONTINENT, while every other SH was from a specific country. It’s a problem because it simplifies the size and diversity of the countries, races, and ethnicity in Africa. This sort of thing is pretty common (like this, for example), which is all the more reason to call it out when it happens.