For 60 years, it’s been nearly impossible for anyone to recreate International Klein Blue. Named after French artist Yves Klein, this specific ultramarine color has shown up in all sorts of pop cultural references from the Blue Man Group to the fashion runways. But why is this specific paint so difficult to make? WSJ explores the story behind IKB and visited British artist Stuart Semple—the creator of paints with colors that mimic icons like Vantablack, Tiffany Blue and Barbie Pink—to learn how he has worked to recreate this coveted blue he calls “Easy Klein” that sells for $35 a tube.