Like said so come. The color of the shellos depends on where it is located along with its shape. So, in East and West of Sinnoh it has two different colors and shapes because it is found in two different areas. The main reason why Shellos was given two forms here is because of Mt. Coronet's long range splitting the region basically in half. So, it is probably that there can be new forms for Shellos in the remakes [if Game Freak sticks to its PokéDex entries, which doesn't always happen].
However, like I said I wouldn't think of it as impossible, but there is a slight chance of it happening and/or not happening. The reason why is because not only of the PokéDex entry, but because a Pokémon does not have to be bounded withinn its specific region. They can be located anywhere and the previous games have proven so. Actually, people say that Groudon, Kyogre, and Rayquaza don't belong in Johto, but yet they are there now. So, saying Shellos or Gastrodon don't belong here is kind of redundant.
Also, here are some of the anime, and DP PokéDex entries I found on Bublapedia:
