William Wrigley Jr. built his chewing-gum company's headquarters in 1921 on the north bank of the river. Graham, Anderson, Probst & White clad it in glazed terra cotta in six shades that grow progressively whiter toward the top, so the tower appears to catch more light as it rises. It was floodlit from the start. The clock tower is modeled on the Giralda in Seville.
Source: Chicago Architecture Center, Wrigley Building; The Wrigley Building, official building history