How is “is the new” used?

In The Intern (2015), one line is: “Gray is the new green.”

Which movie is “is the new” from?

The clip at the top of this page is from The Intern (2015).

Gray is the new green.

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イディオム

used to say that sth has become very fashionable and can be thought of as replacing sth else

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… is the new…
used to say that sth has become very fashionable and can be thought of as replacing sth else
Brown is the new black.

定義：Oxford Advanced Learner's Dictionary 第 9 版