Shinies

A shiny pokemon is a specific Pokémon with different coloration to what is usual for its species. It is one of the many differences that a Pokémon can have within its species. The term "Shiny Pokémon" refers to the sparkling sound effect and animation made at the start of an encounter with one in the games. However, official sources have used the term Shiny.[1]
Shiny Pokémon in the games may differ in color from their normal counterparts either very little or very much. Usually the former is the case, with some Pokémon, such as Glaceon, having their Shiny form be only a few shades lighter in color. However, many Pokémon will have a spectacular difference between their normal and Shiny variations; even extremely common Pokémon like Caterpie show a dramatic difference. An evolutionary line is not necessarily guaranteed to have similar alternate colorations, even if their standard colorations remain consistent; both Ponyta and Rapidash have orange flames, but an alternate colored Ponyta has blue flames, while an alternate colored Rapidash has gray flames.
Despite being completely different colors, however, the two forms of Shellos and Gastrodon are not normal/Shiny variations of one another. Their difference is more akin to the difference between Unown or Spinda.