If you press the escape key while in-game you will see the Unlock Content button. Press that and enter any of the following codes to unlock an item set. Use the "skin item" and a trader wil pop up. Buy and use the dye/potion.

Code	Effect
123456	Girl Skin.
12345	Girl Skin.
123	Male and Female Skin.
274267	"Shinobi Shozoku" set.
373624	"Armament of Gaul" set.
437626	"Armament of Germania" set.
925678	"Garb of the Great Merchant" set.