Why was One Piece white washed?
Well, Zoro did spend those two years in a dark island/gloomy castle, so he probably didn't see much sun. Robin also spent her time in the mountains, I believe, which were hardly places to get sun tans (compared to her old job in a desert).

I don't think the skin pigment changes had anything to do with racism, as these kinds of changes were more associated with tans and less with race.