Pokémon Brilliant Diamond and Shining Pearl introduced a new mechanic to the Pokémon series, which are variations depending on where you encounter them in Sinnoh. This is evident early on in the Pokémon Shellos and its evolution Gastrodon.
Near the beginning of Pokémon Brilliant Diamond and Shining Pearl, players will encounter pink Shellos. Later on, they will encounter 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.
Each Pokémon has the same stats and move set, so the only difference between the two is their appearance, meaning you can catch either if you plan to use 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 the pink Shellos first, making it more common than the blue variant. But don't let that decide which version you use; pick the one you like the most!
For more information on Pokémon, check out How to catch Gible in Pokémon Brilliant Diamond and Shining Pearl and What are the Shiny Odds in Pokémon Brilliant Diamond and Shining Pearl? on Pro Game Guides.