Shellos is one of the main and strongest water type Pokemon.
As you can see there are two different types of Shellos and Gastrodon's; the west sea and the east sea.
The west sea only contains the Pink Shellos and Pink Gastrodon. The east sea only has the Blue Shellos and Gastrodon.
Mt. Coronet seprates the Shellos and Gastrodon from each other.
You can never find both colour Shellos or Gastrodon in the same place. Both Shellos look completely different, with their back, head, and colour.
The head shape differs from each Shellos, making them look unique in their own way.
The back pattern and shape on their backs also differs from Shellos to Shellos.
When they evolve into Gastrodon they both look different and keep the colour from whatever sea they were from; Their head shape and back changes yet again.
You've got to remember that whatever Shellos/Gastrodon you capture will always stay with that same colour; they will never look exactly the same as their cousins.
What We Have here in the above images are the East Sea Shellos & Gastrdon.
Here we have the lovely West Sea inhabitiants Shellos & Gastrodon.