From Thriller Bark arc.

I waited one year to be sure about the color of the dress! Black would be great, the cosplays of this outfit made in black are so classy. Then, the anime showed us a purple dress and the manga, a (horrible) shade of pink.
I choose purple velvet for the dress and boots and painted the lace (unfortunaly, it ended up lilac instead of purple).

Reference image: http://i256.photobucket.com/albums/hh174/nicomeg/bark.jpg