Agnes Martin,” a retrospective at the Guggenheim Museum of the Canadian-born painter who died in 2004 at the age of ninety-two, has the misfortune of being mounted concurrently with “Mark Rothko; Dark Palette,” an exhibition at the Twenty-fifth Street branch of Pace Gallery. The comparison between Martin and Rothko would be inescapable even if the shows weren’t simultaneously on display. Both painters pursued an art of distillation, exploring just how much could be jettisoned from the art of painting without altogether relinquishing its particulars. Martin was vocal in her admiration of Rothko, extolling how he had “reached zero so that nothing could stand in the way of truth.” Her early work, with its sparely applied geometries and gently stated means, owes a clear debt to Rothko’s mature paintings: those hovering fields of color that radiate heat, light, and mystery. At earl ...