A lot of people would argue that Safer is the correct translation, aluding to the fact Sephiroth's name has it's roots in Judiasm and the Qabbalah, as does Safer. But as there's no official word from Square, and he DOES have the appearance that suggests Seraph, it could go either way. But, old argument
Back on topic, it probably has to do with Amano originally designing the character concepts with blonde hair. Of course, he draws nearly EVERY Final Fantasy character with blonde hair, so when it comes time to implement them into the game, it's easy to see why certain characters get colors changed. This doesn't happen so much anymore since Nomura and/or other team members do the concepts for the characters, while Amano seems to only do official box artwork.
Personally, with today's technology I'd like to see Square create a game entirely based on Amano's intricate artwork, that really is only now possible since his detail level is so high. Or better yet a modern remake of FF6 using Amano's designs. The Magitek Armor would rule. Too bad that'll never happen.
