I've had some luck with the Lemon Pledge, but it doesn't last. I settled for doing nothing and let my NT have that "experienced" look.
After eight months the painted sections still look as good as the day I painted them.I have noticed that also and if the same material was use on all the black pieces there would be no fading. The fading was especially bad on the rear facing part of the bike, my bike spent a lot of time parked by a open south facing garage door.
I painted the rear fender and the tail piece a couple of days ago with VHT high temperature for Plastic Paint (available at Auto Zone) and it looks good, time will tell how well it holds up.
It is the plasticizer that give the plastic components their flexibility and to some degree their color. When you do the heat gun/torch trick outlined above, that brings out the deeply imbedded plasticizers to the surface to make the plastic appear new. This only works a time or two as once the plasticizers are "bloomed" out of the plastic, they are gone forever and cannot be restored. The plastc then turns brittle and crumbly. The only solution then is a paint that seals over the plastic part and give the part the new color. The underlying part while improved in appearance is no longer as flexible as when first molded.If the black plastic is severely grayed from the sun I believe it changes the composition of the plastic and thus is difficult to restore to like new with chemicals. Catching it early you may have better luck and can at least protect what is left.
Mine sat out uncovered at work parking lot in the strong Arkansas sun. The tail piece especially did not look or FEEL anything like a new one. It had been changed into a different material for all intents and purposes. I eventually replaced the inexpensive pieces.
Arknt