As Beamishken said, they're likely polyurethane tubes and a good scrub to remove the oxidized surface is the first step. If you get the oxidised surface removed you can seal the surface with floor polish. Do a search because there has been a number of discussions on the forum over the years. One of the polish products was sold in supermarkets under the me Klear, iirc it was made by Johnson but it had a name change which I can't remember. There are others though and I buy mine on eBay 5ltr at a time. A couple of coats spread on with a cloth works well and is fairly permanent.
However, it won't bring back the colour. I've seen very badly bleached polyurethane tubes, especially the top surface.