The nail-polish that the Akatsuki wear changes in colour through out the anime. Purple, black and light purpley-blue are all accepted as the Akatsuki nail-polish.
Correction, they all wear different colors. Deidara wears black, Pein/Pain wears red, Itachi wears the gothic blackish purple color, etc. I've attacked a link to a guide someone made to show the colors quite accurately, but I recommend looking up the specific character's pictures for closeups of their hands if they're not 100% confident in the guide (which does have screenshots of their hands). http://img05.deviantart.net/5f39/i/2...ha-d5yxs5t.jpg