A "Shiny Mew" in Pokémon is a rare alternate color version of Mew. While a normal Mew is pink in color, its shiny variant is colored a light blue. Shiny Pokémon first appear in the Generation II versions of the game that was released on the Game Boy Color.
The "Shiny Mew" is the only Pokémon in the Generation II games that cannot be obtained in its shiny variant, though it is available from Generation III onwards. In Pokémon lore, Mew is said to possess a DNA that contains genetic information from all existing Pokémon. This allows it to transform into any Pokémon and use all of their techniques.