Actually, I just found an explanation:
The Yeo portrait
If the first official portrait of the King to be unveiled since his Coronation is anything to go by, symbols of his devotion to ecological causes are likely to remain a leading leitmotif of royal iconography throughout his reign. The larger-than-life likeness, which measures 8ft 6in by 6ft 6in, is the work of British artist Jonathan Yeo, who has previously painted the King's father, Prince Philip, the late Duke of Edinburgh, and Charles's wife, Queen Camilla. Here, the uniformed figure of Charles appears to be emerging from a heavy haze โ a vivid vermillion mist that tinges everything save the King's incongruously serene face, his sympathetic hands (which rest on his sword's pommel), and a small monarch butterfly about to land on his sovereign shoulder.