What does the Money Heist costume mean?

The crew wears these costumes for a simple reason:

The Salvador DalĂ­ Mask

When the famed Spanish artist was alive, much of his work was created during Zurich's Dada movement, which was about rejecting the modern capitalist society.

The Red Coverall

The red uniform has a practical purpose, of course. 

The thieves can cover themselves up completely, rendering them unidentifiable to both the hostages and the police.

The costume has been used in actual political protests.

In season 3, the Professor sits the thieves down and speaks about how their red costume is being used in protests all around the world.

You can buy your own Money Heist costume.

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Money Heist makes for an easy Halloween costume. Given the show's popularity, pre-made costumes are sold.