Have you looked at patterns on public seating wherever there's fabric? Or e.g. hotel carpets and the like? Consider the heavily patterned designs often serve a function.

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.

A common approach. Many of us wear blue and black jeans, few white.

The women who wear white tend to choose fabric that doesn't stain easily, an approach TfL might have considered earlier.

No surprise you're taking the least charitable view possible.

Perhaps what they're doing is cleaning and then looking for a way to mitigate the need for intensive future cleaning. Seems sensible.

Unfortunately, the material they chose for the station walls is a sort of porous cement that is likely uncleanable.

Also hostile architecture - benches with no backrests.

The benches are designed to be used for on average 4-5mins, maybe 20mins if having to wait for a special train. Their purpose is to serve commuters using the trains. Not as an alternative to proper social housing (which the UK has lots of).

The wall is the backrest.

Except the wall curves inward, which pushes your head, neck and upper back forward. Having sat on them, it's rather uncomfortable, and must be worse for people taller than me. That said, I'm not convinced it's deliberately hostile, I just think it's incompetent design, favouring a minimalist style over functionality.

Reminds me of modern API layers bolted on top of crumbling legacy systems

Funnily, the same approach is often used for acoustic / noise issues. Add music / other sources of sound to try and mask the acoustics issue (unsuccessfully).

Isn't it the basis of active noise cancelling? It's pretty successful.

Yes and no. Active noise cancelling works well at a point and against some common kinds of noise, but if you want to act on a large area such as a tube station, active noise cancelling won't work. Much better to introduce a bit of white noise or music, that makes e.g. the clack-clack-clack of footsteps on the stairs much less noticeable in the wider area.

No, active noise cancelling isn't about masking noise with sound, active noise cancelling takes the original noise and mixes it back 180 degrees out of phase. The net result is zero noise, when done perfectly. In practice it's almost like that (for the frequencies covered by the noise cancelling equipment - in an airplane it's like the background noise just.. turns off. What's left is typically people talking. It's easier to hear the flight attendants).

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 would guess that the kind of plastic resin that's used for the walls reacts with heat and can easily be permanently marked.

It's glass fibre reinforced concrete (GFRC). Very much doubt it's heat-reactive, suspect it's just oil & dirt on a somewhat porous surface (so cleaning properly is labour-intensive, stickers are cheaper)

It is very very very unlikely that they would build an underground pedestrian station that had walls that reacted to heat, unless it was to absorb heat in case of fire.

I'd also think that plastic resins are not the best thing to be reacting to heat.

Or replace the material with one that is easier to clean, which is exactly what these vinyl stickers do.