Fender or shore power cable cleaners are worth a try.
There are paint like coatings that are available. You need to remove the dorade, tape it off and follow the instructions.
Beyond that, it's "buy new ones" time.
P.S. - IN the future, you can wax them to protect the surface from absorbing stains and dirt, Cleaning them with boat soap and a SOFT brush and sponge occasionally is always helpful. My Vetus dorades have "caps" on them made out of sunbrella.