From Bulbapedia, the community-driven Pokémon encyclopedia.
In Generation II, whether or not a Pokémon is Shiny is determined by its IVs.
A Pokémon is Shiny if and only if both of the following conditions are met.
*Its Defense, Speed, and Special IVs are all 10.
*Its Attack IV is 2, 3, 6, 7, 10, 11, 14, or 15.