Hi Jhance,
One of the difficulties with plastic rubrails on pleasure boats is there is little knowledge of exactly what formulation of "plastic" was used to fabricate the rubrails. Some manufacturers use PVC, some UHMW, most use "whotheheckknows". And unless you can locate the OEM manufacturer of the rails, even the boatbuilders don't know. If you're lucky enough that your boatbuilder is still in business, and his records are up to snuff, and he's kind enough to share the OEM supplier of the rubrail, and that OEM is still in business, you might be able to track down the original material.
THEN, trying to locate an cleaning/coloring/restoring agent to "bring back the black" for that particular material is a further goose chase. AND THEN the results of that restoration requires some time (days, weeks?) to tell if the color restoration will hold.
Bottom line, if you find an agent that works with YOUR rails, please share on this forum. So far, I've been unsuccessful in exactly the same quest on my 2000-vintage Pacific Trawler 40. My best results to date have been to color my rubrails with black Magic Marker! Lasted all of about 3 weeks.
Regards,
Pete