Wouldnt a "good" vader still cad in black make people around him tremble in fear and remember all the things he did? A outfit turned white would make people think he's no longer the bad person he once was...I mean, black vs.white is symbolizm I think....black has always been the bad guy and white always the good guy..(except Will Smith in MIB, the good guys always dress in black)
Have you not heard of the white knight? I think any other color would have been bad...since he couldnt remove the life sustaining armor, he needed to do something to show his new way of thinking.
is making a white costume a gay idea? well, I doubt very many fans out there would know why it was done, and 90% would think it was stupid looking, but up to that point, no one had done it, so being original is still worth the applause.