Have heard of people using a hot air gun to get them black again! Not sure how etc.. The very nice MaxPower GT/G40 had this done and looked damn good afterwards.
I've always used Heat Gun or Autoglym green gel.
The gel is messy and fades as it is a chemical reaction.
The heat gun causes a physical reaction on the surface of the bumper, the oxidisation / fading is effectively returned to its original state (black)
You just have to be careful, if worried practice with a few spare bumpers.
The trick is to gently heat it and keep an eye on it.
As it warms up you can see it returning to black, too far though and the bumper starts to bubble.
After a few bumpers you get a good feel and it becomes easy.
Cheers
Stuart