Why is Nico Robin not tan?

Explanation of Nico Robin's Skin Color

According to Francis Roussell Fangonilo, Nico Robin's color palette remained consistent throughout the series. The coloring choice for her character was meant to represent her post-timeskip look. The initial color discrepancy was simply a mistake by the anime team, which was rectified during the character redesign for the timeskip. Additionally, her lighter complexion aligns with the fact that she no longer resides in Alabasta.