Skin Pieces
From Galaxy Attack: Alien Shooter Wiki
Skin Pieces can be obtained in the Matrix or purchased in the Clan Market.
Once you collect all the required pieces for a skin (you can see how many you have in the Stack), you will unlock that skin and be able to select it whenever you pick up that ship.
Any Skin Pieces you collect for a particular ship type will give you an additional damage bonus for all other skins of that particular ship.
For example, if you own Thunder Blade and Thunder Blade Celadon and have collected 3 skins for Galaxy Thunder Blade and 6 skins for Thunder Blade Cerulean, you will get a bonus of 5.4% damage for the two ships you own. You can see this by clicking on the evolve stone count of the spaceship screen.