The Rainbow Fish was a legendary creature that was a large as a whale. This great fish swallowed Buddha, an incarnation of the deity Vishnu. Vishnu occasionally descends to Earth to help humanity fight evil. In order to do so, he takes avatars or incarnations. The main ten avatars of Vishnu are thought to be:

Fish: Matsya
Tortoise: Kurma
Boar: Varaha
Man/Lion: Narasimha
Dwarf: Vamana (the first fully human)
Parasurama
Rama
Krishna
Buddha
Kalkin

Since Vishnu was a god, the mythical giant fish did not survive. The fisherman that caught The Rainbow Fish freed Buddha and the fish provided food to the entire nation for one full year.

The scales of the rainbow fish were green, blue, yellow, and red:

Green scales: made of grass - represent Earth or Prithvi. Earth is found in all objects, animate and inanimate, including the human body. Its character paramanu (the atom) is eternal and its character karya (work) is perishable.

Blue scales: made of ice - represent water or Jal. Water also has an eternal or atomic character and a perishable one, which you can see in rain, rivers and other bodies of water.

Yellow scales: made of lightning - represent air or Vayu. Air is that which can be felt when you breathe. The Puranas mention 49 types of air. The atoms of air are eternal, while its manifestations in winds and storms are temporary.

Red scales: made of flames - represent fire or Agni. Fire also has an eternal and a perishable side.