I don't have evidence but I always assumed they do it to mask wear and dirt - the odd geometrical patterns make it less obvious how old and dirty they are, compared to a single clean colour.

They're great! Someone was selling t-shirts with the patterns a while back but TfL hold copyrights on them and the seller was C&Ded.

Here's my favourite, from the best tube line in London: https://dribbble.com/shots/1642243-Tubepapers-Victoria?list=...

I don't even know why they have seat cushions to begin with. It just seems like they're breeding grounds for bacteria and would be difficult to properly clean.

NYC has only hard plastic chairs.