I was working on my xk150's engine several weeks ago. Several engine plastic pieces (intake tube in particular) were originally black but due to age, heat, etc they were now a very light grey. At the same time I was applying an old product I've used before on the windshield and thought, what the heck. I applied a few drops to the plastic and it revived the color. I wasn't convinced it would last but the following day it was still very black. I then proceeded to completely slather the exterior of several pieces with the gel product and let it sit overnight. I appeared as if some of it either evaporated or further penetrated the plastic. I wiped it clean and reinstalled. It is still quite black weeks later and if it remains that way for some months I think I'll continue to use it on some visible panels which are showing their age.
So...long winded I know, sorry. Today I remembered that the red push buttons on my xk100 seat belts were pink...I applied a few drops and rubbed it in. This is the result.






wj

 

It's probably a polysilane.