Susan Mansfield on Jonathan Yeo's portrait of King Charles

The effect of the colour is that the King’s uniform – that of the Welsh Guards, in which he is a Regimental Colonel – seems to merge with the backdrop, pushing the King’s head and hands forward. It’s a portrait which calls our attention to the sitter first and foremost as a human being.

When the Drapers Company commissioned Yeo, Charles was Prince of Wales, but the painting grew in significance when it became clear that it would be the first official portrait to be unveiled after Charles was crowned King.

Compared with some of the more experimental artists who have painted members of the royal family in the past, Yeo, who is the son of the former Tory MP Tim Yeo, could be seen as a safe pair of hands. A highly sought-after contemporary portrait painter, he has painted both Prince Philip and Queen Camilla (when she was Duchess of Cornwall) and was commissioned by the Queen to paint Sir David Attenborough.

However, he has also shown a determination to do things his own way, painting a full-frontal nude portrait of Ivan Massow, then chairman of the ICA (Institute for Contemporary Art), and making an unauthorised portrait of George W Bush from collaged cuttings from pornographic magazines. His portrait of Tony Blair – a weary figure wearing a red poppy – is thought to be a reference to the war in Iraq.

Artists who paint royalty know they are walking in the steps of esteemed forebears, artists who helped shape the public image of a monarch as Hans Holbein the Younger did for Henry VIII, or Velazquez did for Philip IV of Spain.

More recent monarchs have chosen to commission portraits by many different artists, perhaps to spread royal patronage more widely. Queen Elizabeth II was the most portrayed woman in history, sitting for more than 1,000 portraits, by artists from Annie Leibovitz to Lucien Freud.

A portrait by Justin Mortimer caused controversy when it was unveiled in 1998 as the Queen’s head appeared to be separated from her body by a bright yellow background. A portrait by Rolf Harris, commissioned by the BBC for her 80th birthday was critically derided, and has not been seen since the entertainer was jailed for sex offences in 2014.

While royal portraitists of the past might have gone to town on symbolism, including objects or colours with particular meanings, Yeo has settled on just one. The butterfly was the King’s own suggestion, and highlights his support of environmental causes. As a “monarch” butterfly, it is also part of another time-honoured portraiture tradition, that of the visual pun.

The size of the painting – a monumental 8ft 6in by 6ft 6in – was determined by the location in which it will hang in Drapers Hall. This, in itself, gives it a kind of grandeur. But it is not seeking to elevate its subject in any way beyond the literal.

This, rather than the swathes of red, is what makes it a brave portrait. It is a frank depiction of its subject, a man in his seventies with thinning hair, a lived-in face and wrinkled hands. It is portrait of a human being on whom the duties of royalty have fallen. He looks the viewer levelly in the eye, carrying his lot with forbearance.