Although not mentioned in the text of the story, the silver costumes are apparently used because their designer, the scientist Ull, lost his fortune in the Depression speculating on silver (and believes he is entitled to get it back by force).
It is implied that the silver costumes were chosen by Ull, despite being unmentioned in the text, as a result of his financial misfortunes during the Great Depression, when he had gambled on silver and lost everything.