Interesting fellow this Cataract guy is. He works alongside Bison during the quest for capturing Sean. So, why the purple skin??? Well, for one...he's actually dead. During the saga of SF, Bison had been chipping away at a side project to find a legendary wrestler (whom I have no name for as of yet) that possesses the ability to successfully defeat his opponents with size and speed. Unfortunately, this wrestler passed away due to heart problems from overusing a steroid that helped him achieve his legendary status. Bison figured this guy would be a great asset to Shadloo even if he had to dig up his body and resurrect him. Plus, it was easy to recruit him without having to force him to join.
In order to resurrect Cataract, Bison passed along some of his Psycho energy into Cataract's body which causes his skin to turn purple. He also injected a good amount of the steroid that Cataract used when he was alive to amplify his strength and speed. Cataract no longer knows how to speak. Instead he murmurs and makes weird noises to express his mood. He is big and FAST and can cause a good amount of damage if you let him get close. You have been forewarned.