They were born that way? XD
I don't think their color scheme has anything to do with their affiliation for light or darkness. Someone can feel free to correct me if I'm wrong but I think Character Designer, Tetsuya Nomura, said that he just likes designing villains with silver hair.
He was the same person who designed Sephiroth in Final Fantasy VII and all the other villains in Final Fantasy VII Advent Children. It's just his "thing".