I don't think Robin's skin tone has changed much manga-wise, considering she's always been pretty white in the manga. Part 1 Robin >>> Part 2 Robin though, especially with either her Miss All Sunday and CP9 outfits.
As for Nami, I probably prefer her in Part 2, though only manga-wise, considering things are more consistent and less exagerrated than in the anime. Her best look was probably the dress she wore during the Capone meeting.