Koi varieties are distinguished by coloration, patterning, and scalation. Some of the major colours are white, black, red, yellow, blue, and cream. While the possible colour variations are virtually limitless, breeders have identified and named a number of specific categories. The most popular category is Gosanke, which is made up of the Kohaku, Taisho Sanshoku, and Showa Sanshoku varieties.

New koi varieties are still being actively developed. Ghost koi were developed in the 1980s, and have become very popular in Cyprus. There are a hybrid of wild carp and Ogon koi, and are distinguished by their metallic scales. Butterfly koi (also known as Long fin koi, or Dragon Carp) were also developed in the 1980s, and are notable for their long and flowing fins. They are hybrids of koi with Asian carp. Butterfly koi and Ghost koi are considered by some to be not true Nishikigoi.

The major named varieties include:

  • Kōhaku (紅白) A white-skinned koi, with large red markings on the top. The name means “red and white;” kohaku was the first ornamental variety to be established in Japan (late 19th century).
  • Taishō Sanshoku (or Taisho Sanke) (大正三色) Very similar to the Kohaku, except for the addition of small black markings called sumi (). This variety was first exhibited in 1914 by the koi breeder, Gonzo Hiroi, during the reign of the Taisho Emperor.[citation needed] In America, the name is often abbreviated to just “Sanke”. The kanji, 三色, may be read as either sanshoku or as sanke.
  • Shōwa Sanshoku (or Showa Sanke) (昭和三色) A black koi with red (hi 赤) and white (shiroji 白地) markings. The first Showa Sanke was exhibited in 1927, during the reign of the Showa Emperor. In America, the name is often abbreviated to just Showa. The amount of shiroji on Showa Sanke has increased in modern times (Kindai Showa 近代昭和), to the point where it can be impossible to distinguish from Taisho Sanke. The kanji, 三色, may be read as either sanshoku or as sanke.
  • Tanchō (丹頂) Any koi with a solitary red patch on its head. The fish may be a Tancho Showa, Tancho Sanke, or even Tancho Goshiki. Named for the Japanese crane (Grus japonensis) which also has a red spot on its head.
  • Asagi (浅葱) A koi that is light blue above and red below. The Japanese name means “light blue.” Sometime incorrectly written as 浅黄 (light yellow).
  • Utsurimono (写り者) A black koi with a white, red, or yellow markings. The oldest attested form is the yellow form, called “Black and white markings” (黒黄斑 Kuro Ki Han) in the 19th century, but renamed Ki Utsuri (黄写り) by Elizaburo Hoshino, an early 20th century koi breeder. The red and white versions are called Hi Utsuri (赤写り) and Shiro Utsuri (白写り)respectively. The word utsuri means to print (the black markings are reminiscent of ink stains).
  • Bekko (鼈甲) A white-, red-, or yellow-skinned koi with small black markings sumi (). The Japanese name means “tortoise shell,” and is also written as べっ甲. The white- red- and yellow varieties are called Shiro Bekko () Aka Bekko () and Ki Bekko () respectively. May be confused with the Utsuri.
  • Goshiki (五色) A black koi with red, white, brown, and blue accents.
  • Shūsui (秋翠) The Japanese name means “Autumn jade.” The Shusui was created in 1910 by Yoshigoro Akiyama, by crossing Japanese Asagi with German mirror carp.[citation needed] The fish has no scales, except for a single line of large mirror scales dorsally, extending from head to tail. The name is sometimes also written as 秋水.
  • Kinginrin (金銀鱗) A koi with metallic scales. The name translates into English as “gold and silver scales.” Often abbreviated to Ginrin. There are Ginrin versions of almost all other varieties of koi, and they are fashionable.
  • Kawarimono (変わり者) A “catch-all” term for koi that cannot be put into one of the other categories. This is a competition category (you cannot buy a “kawarigoi” from a pet shop) and many new varieties of koi compete against each other within this one category. Also known as kawarigoi (変わり鯉)[citation needed]
  • Ōgon (黄金) A koi of one color only, but with metallic scales (ginrin 銀鱗 or hikarimono 光者). The most commonly encountered colours are gold, platinum, and orange. Cream specimens exist but are very rare. Ogon compete in the Kawarimono category and the Japanese name means “Gold.” The variety was created by Sawata Aoki in 1946 from wild carp he caught in 1921.
  • Kumonryū (九紋竜) Kumonryu is a black fish with curling white markings. The patterns are thought to be reminiscent of Japanese ink paintings of dragons. They famously change colour with the seasons. Kumonryu compete in the Kawarimono category.
  • Doitsu-goi (ドイツ鯉) A German mirror carp with only a single line of large mirror-like scales along the top. Also written as 独逸鯉.
  • Ochiba (落葉) A light blue koi with yellow patches, reminiscent of autumn leaves on water. The Japanese name means “fallen leaves.”
  • Koromo () Koi with patches of blue- or black-edged scales. This variety first arose in the 1950s as a cross between a Kohaku and an Asagi.[citation needed] The most commonly encountered Koromo is an Ai Goromo, which is coloured like a Kohaku, except that each of the scales within the red patches has a blue- or black-edge to it.
  • Hikari-moyomono (光模樣者) A koi with coloured markings over a metallic base, or koi in two metallic colours.
  • Ghost koi – A hybrid of Ogon and wild carp with metallic scales. Considered by some to be not Nishikigoi.
  • Butterfly koi – A hybrid of koi and Asian carp with long flowing fins. Various colorations depending on the koi stock used to hybrid. Considered by some to be not Nishikigoi.