I’ve wondered the same thing. My theory is to make it look like grass.

I once read that it was a type of paint intended to waterproof the cement roofs, and that green initially was the cheapest color to make it, and see that it had been applied properly. (I cannot even remember where I read this, so maybe it is wrong, but it sounded plausible)

It might be what the Japanese do: they literally build their buildings in a way that is sustainable and can grow and keep grass, among other flowers if desired, on its own… Apparently, that’s a thing