You step through the heavy wooden doors and stop. The light inside La Llotja falls differently today. The six spiralling columns still twist upward toward the vaulted ceiling, just as they have for nearly six centuries. But something has changed entirely. Color floods the room. It moves across stone, spreads onto the entire floor, pools in shadows, and erupts where you least expect it. Katharina Grosse’s exhibition “Arrels” (Catalan for roots) doesn’t hang on the walls of La Llotja. It…