One Piece - The Pirate Alliance

Well he wasnt lying.

No, what happened was that Oda had originally intended Robin's skin to be a lighter color, but the animators, at Robin's debut in the anime, made a mistake and made her skin darker. Instead of changing it right away they decided to wait until the timeskip.
Proof: old manga cover

For the record I wish that Robin's was always dark. She looks way hotter anyway.