I just wanted to chime in with my retrobright experience. While it looked great at first (maybe 90%), my 4000 faceplate also yellowed again over the period of a few years. I know this works for others, but I recently tried the "leave it in the sun for a few hours" approach and was horrified to find it looking the color of a used cigarette filter, significantly worse than ever.
I think fralbore's idea is stellar. (The Alfa is pretty stellar, too!

) Plastic paints act somewhat more like a dye, and will be a permanent fix while leaving the original sheen and texture intact (assuming a good match with regard to sheen).
The only caveat might be the faceplate badge: I'm not sure how easy they are to safely remove, and this could be an even bigger downer when your yellowed faceplate has an original pristine badge with its transfer tape still intact.
This outfit however appears to sell reasonably accurate looking replacements:
www.plakativ-druck.ch