Mark Rothko’s monumental canvases of floating color fields were representations, he said, of different emotional states. Two years before he died, in 1968, he was diagnosed with an aortic aneurysm and was told to stop painting on such a large scale. With illness came discovery. He switched from oils to acrylics, and from canvas to paper, and worked on a much smaller scale. From the deepest pink to the most evocative blue, these works are luminous jewels. “1968: Clearing Away” focuses on Rothko’s work of the late 60s. —E.C.