Many people always wonder why they made Robin white after the timeskip. Well Robin was never dark-skinned in the manga. That was an anime-only decision but she was always white, canonically speaking.
Robin was always supposed to be light-skinned. What happened was that Oda had not produced a colored image of her before she first appeared in the anime, and so Toei made a guess about her skin color that turned out to be wrong. In addition, Robin’s eyes were supposed to be brown, but the anime made them blue instead. This also happened with Tashigi, who was originally supposed to have black hair instead of blue.
Oda did not whitewash Robin, she has always been canonically light-skinned. Oda basically uses the same skin colour for the crew in the manga, just check some manga covers featuring the Straw Hats.
There is possibility that the animators just wanted to get closer to canon. When they realized their mistake, they realized that they couldn’t very well just suddenly change it between episodes, so instead they waited until after the timeskip, since everyone’s character designs were going through some changes anyway. Robin losing her tan was no more strange than Ussop’s gains, Zoro’s facial scar or Franky’s rebuild.
Anyway it’s pretty easy to justify the color change. Robin was first introduced after having spent several years living on a desert island; it’s not all that surprising that she would have a tan. She then spent part of two years on a winter island, and most likely spent most of the rest indoors studying; it would make sense that she would lose that tan simply due a lack of effort in maintaining it.
In an SBS question, a fan asked if the Straw Hats were real, what nationality would they be of, based on their appearances. Well, Oda’s answer for Robin was Russia. I don’t know about you, but I doubt there are many black people from Russia.