Generation II
In Generation II, Shininess is determined by the individual values (IVs) of a Pokémon. If a Pokémon's Speed, Defense, and Special IVs are 10, and its Attack IV is 2, 3, 6, 7, 10, 11, 14 or 15, it will be Shiny. This determination allowed for compatibility with Generation I, as if a Shiny Pokémon is traded back, then traded forward, it would retain its Shininess. Despite the bonus of backward compatibility allowing for a Shiny Pokémon to learn Generation I TMs, provided it was a Generation I Pokémon, Shiny Pokémon are only slightly above average in terms of IVs.
Due to the probability of the IVs lining up in any of these manners to produce a Shiny Pokémon, the chance of obtaining one in Generation II is approximately 1/8192. However, if one breeds a Shiny Pokémon properly, the offspring may have as high as a 1/64 chance of being Shiny, depending on the species of Pokémon.
Shininess is inherent to an individual Pokémon; that is, a Pokémon that starts out Shiny will always be Shiny, and one that is not will never be (provided its data is not altered through cheating). Upon evolution, a Pokémon will retain its Shiny status. A Shiny Charmander, if leveled up, will eventually become a Shiny Charmeleon and then a Shiny Charizard, just as a regular Charmander will become a regular Charmeleon and then a regular Charizard. This is due to the status being determined by values inherent to the Pokémon; however, there is a difference in the determination between Generation II and Generations III to IV.
To highlight alternate coloration, a red Gyarados at the Lake of Rage is part of the plot of the Generation II games and their remakes. After its defeat or capture, a Red Scale will be obtained, which can be given to Mr. Pokémon in exchange for an Exp. Share. The red Gyarados is the only wild Pokémon in the games that will be Shiny without fail.
The Pokémon that hatches from the Odd Egg in Crystal has a 50% chance of the result being Shiny.
