Crossword Genius

Hans van Meegeren was proved to be a forger of Vermeer by his anachronistic use of ____ blue (6)

Ross

I believe the answer is:

cobalt

(Other definitions for cobalt that I've seen before include "Deep blue pigment" , "kind of blue" , "Metallic element, Co" , "Magnetic metal" , "Metal giving its name to type of deep blue" .)

I've seen this clue in The Sunday Times and The Times.
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