[jychang]

Well, the counterargument is that in theory, you can imagine a way to create structural color regardless of substrate. So imagine a technology that shines a laser on a car or a block of concrete and makes it blue; I'd argue that's correctly "without chemicals".

Of course, I doubt you can do that to any random substrate, since the color will depend on the properties of the material.

[shultays]

  So imagine a technology that shines a laser on a car or a block of concrete and makes it blue

There is something like that for sheets of steel at least https://www.youtube.com/watch?v=2ncEfAxkuFA

And here is a video explains it https://www.youtube.com/watch?v=RsGHr7dXLuI