Red: Its hard shell repels any kind of attack. It is vulnerable only when its shell is open.Yellow: The shell can withstand any attack. However, when it is open, the tender body is exposed.Gold: It swims facing backward by open ing and closing its two-piece shell. It is surprisingly fast.
Prof. Lotus' Notes: When Shellder clamps on to a Slowpoke tail, they both evolve into a Slowbro. When it does the same on the head, Slowpoke will evolve into Slowking. Slowking is considered one of the most intelligent creatures in the Pokemon universe. This further strenthens the idea that they have a symbiotic relationship. Cloyster's shiny form is Golden in colour I believe this is because of Cloyster's ties to luxury. You'll find a lot of Golden Oyster Shell trinkets.