One of my customers had a company come round and apply something to their faded mahogany effect UPVC frames that appeared to restore the depth of colour back to them, but it only lasted a few months and they were back to the faded look. It also made a terrible mess of the windows (like grease that didn't want to come off) I think they sprayed it on and then wiped it off.
If there is something out there that can be used to restore faded mahogany effect UPVC without making a mess of the windows I'd like to know about it too.