How do you repair yellowed or faded Lego blocks?
When I first got my kids some Lego I did it by bulk buying 25 kilo bags of used bricks off the Internet. Now these bricks had obviously been in outdoor sandpits, chewed by dogs, hidden under carpets - you name it.

We successfully restored almost all of them by by washing them in a bath full of a Milton solution, scrubbing each one with an old toothbrush and then leaving them to dry on a towel. 

This cleared up some yellowing that we had thought was sun damage and fading, however there were some (especially some black pieces) that just looked off-grey and no scrubbing sorted them.

I would suggest trying this method, but if it is sun damage that is more than the outer surface I think you might not have any success.