I'm curious how they were able to patent a technique invented by nature millions of years ago.

the displays have an array of switchable mirrors individually addressable, unlike nature in this case.

(but sort of like chromophores in an octopus or cuttlefish, perhaps).

I see, but those MEMs mirrors were already invented.

Inventions can be useful recombinations or applications of other inventions. They don't need to be wholly unique unto themselves. Indeed, the vast majority of them are not wholly unique.

We're talking about a millions year old invention here.

As far as the patent system is concerned, 20 years and a million years are the same thing. If you combine some million year old things in a new way, you can get a patent.