The plastic piece below the windscreen, and plastic around the side mirrors, on my 1990 XJ40 had faded to grey/white in a typically blotchy and unsightly way. There are lots of black plastic dyes that claim to last a few months, but Forever Black claims to last "Forever" and people talk well of it in Web forums so I picked that to try.
It worked well, although it's only been a week since I applied it so longevity remains to be seen. On one of the forums someone warned that they acidentally got it stuck on some chrome trim and it made that black as well. So I thought it might be great for replacing the original black coating over parts of the window trim at the back of the rear side windows. The original black coating there had cracked and flaked away entirely from the little aluminium triangle pieces, and was half-gone from over the chrome as well.
It doesn't paint onto metal well, you just get streaks, but you can push on the sponge to apply a bubbly layer over the surface. That way I got it applied fairly well, and once dry it left a very nice matt finish. It will scrape off, but only with some effort. However on the aluminium a couple of tiny chips have already come off so maybe it will crack away from there quickly. But paint should work on that anyway, and so far it's holding up on the chrome.