Pokémon Brilliant Diamond and Shining Pearl introduced a new mechanic to the Pokémon series, which varies depending on where you encounter them in Sinnoh. This is evident from the beginning of the Pokémon Shellos and its evolution Gastrodon.
Towards the start of Pokémon Brilliant Diamond and Shining Pearl, players will encounter a pink Shellos. Later, they will encounter a blue Shellos. Both of these Shellos will evolve into the color variant that matches their first form. Despite their different color and appearance, there is no difference between these Shellos and Gastrodons.
Related: Where to find Charmander, Squirtle, and Bulbasaur in Pokémon Brilliant Diamond and Shining Pearl
Each Pokémon has the same stats and moves, so the only difference between the two is their appearance, which means you can catch either one if you plan on using one on your team. There is also no difference in the Pokédex, so you can catch either to complete your Pokédex.
Most players will encounter pink Shellos first, making them more common than the blue variant. But don't let that decide which version you use, choose the one you like the most!
For more Pokémon information, see How to Catch Gible in Pokémon Shining Diamond and Shining Pearl and What Are the Shining Odds in Pokémon Shining Diamond and Shining Pearl? on Pro Game Guides.
