In wikipedia, Koi (鯉?, English /ˈkɔɪ/, Japanese: [koꜜi]) or more specifically nishikigoi (錦鯉?, [niɕi̥kiꜜɡo.i], literally "brocaded carp"), are a group of fish that are ornamental varieties of domesticated common carp (Cyprinus carpio) that are kept for decorative purposes in outdoor koi ponds or water gardens.
Koi varieties are distinguished by coloration, patterning, and scalation. Some of the major colors are white, black, red, yellow, blue, and cream. The most popular category of koi is the Gosanke, which is made up of the Kohaku, Taisho Sanshoku, and Showa Sanshoku varieties.
However, this description is wrong a little. A blue one is rare. This book and the following picture will give you a clear idea of color of KOI.
