How do you clean a power cord? I know, thread drift.
The absolute best way to clean a ground power cord--- and we tried all sorts of things from citrus cleaners to "purpose made" cable and fender cleaners--- is GoJo. The original version with pumice in it. The one without pumice, or the citrus-y orange version they make, doesn't cut it in our experience.
This was actually a tip from a former member of this forum from several years ago. We tried it, and it's terrific.
It's also ideal for cleaning off dirty, scummy fenders.
Now if the cord or fender material is actually stained--- something has gotten down inside the rubber or vinyl or whatever they're made of--- pretty much nothing will lift that out except maybe weathering or UV or whatever as described by RT.
But if the problem is gooey, sticky dirt that's built up over time which was the case with our cable, GoJo takes that off completely. It's not a wipe-on, wipe-off process. It takes some effort to do it. But it works and it leaves no stickiness behind, nor does it attack the cable coating and make it a bit tacky, which simply attracts dirt again.
GoJo (with pumice) is terrific. A number of years ago we (Boeing) started using it in the washrooms in all our manufacturing and assembly buildings.