A zabuton which a rakugo storyteller uses on the sage is called koza-zabuton. The koza-zabuton is placed on the dais in the center of the stage.
Since the rakugo storyteller often makes lots of dramatic gestures during the performance, the large-sized zabuton is used as a koza-zabuton so that the rakugo storyteller can concentrate on the performance without sliding down the zabuton.
Originally, the dais (koza) was used by a person of high rank such as the emperor, shogun or nobleman. For this reason, so purple, which is a noble color, is used for the koza-zabuton.