I'm pretty sure each of their nails match the color of their ring, and the rings are only given once you become a member.
From Narutopedia:
Akatsuki Rings and Ring Positions
What seems to be most important, however, are their rings. In total, there are ten rings given to each of the ten main members of Akatsuki. The rings apparently signify one's membership and ring position, since when Orochimaru defected, he took his ring with him[17] and as such wasn't replaced.[18] The rings' exact purpose have not been revealed.
The ring positions of the members.
The various rings are:[19]
Right thumb: 零 (rei, "zero"); worn by Pain. Its colour is purplish-grey.
Right index finger: 青 (Ao, shō, "blue," "green"); worn by Deidara. Its colour is teal.
Right middle finger: 白 (bya, "white"); worn by Konan. Its colour is white.
Right ring finger: 朱 (shu, "vermilion," "scarlet"); worn by Itachi Uchiha. Its colour is red.
Right little finger: 亥 (gai, "sign of the boar"); worn by Zetsu. Its colour is green.
Left little finger: 空 (kū, "sky," "void"); worn by Orochimaru. Its colour is slate blue.
Left ring finger: 南 (nan, "south"); worn by Kisame Hoshigaki. Its colour is yellow.
Left middle finger: 北 (hoku, "north"); worn by Kakuzu. Its colour is dark green.
Left index finger: 三 (san, "three"); worn by Hidan. Its colour is orange.
Left thumb: 玉 (gyoku, "jewel," "ball," also the black king in shogi); worn by Sasori and later by Tobi. Its colour is purple.
