Why is koi fish called a ring?

One kind of koi fish is called mustard koi fish instead of ring. Mustard koi fish is a rare variety in koi fish, and its main feature is color. Koi fish, a young mustard seed, is similar to koi fish, so it is difficult to distinguish it. Koi fish, a mustard seed, gradually turns pale green in background color and pale pink in scales in adulthood. Compared with noble and bright gold and koi fish, mustard seed koi fish is more restrained and moist.