You can regularly clean well enough to address health and safety and still end up with plenty of marks that are high effort to remove that only really affect appearance, and so designs that reduce the visual impact have been used in high traffic spaces pretty much "forever" as a means to reduce the need for high effort cleaning.
Not really much difference for a wall.
The women who wear white tend to choose fabric that doesn't stain easily, an approach TfL might have considered earlier.
Perhaps what they're doing is cleaning and then looking for a way to mitigate the need for intensive future cleaning. Seems sensible.
If instead it was by adding sound then it would defy the purpose. I use active noise cancelling because if I don't I'll hear a noise in my head for weeks after a long-haul flight (I have tinnitus in one ear which makes it even worse).
I'd also think that plastic resins are not the best thing to be reacting to heat.