He also visited a project linked to his Earthshot environmental prize which creates a type of sustainable dye that can reduce the fashion industry’s use of harmful chemicals – so colours can really be green.
Catherine did not appear at Royal Ascot earlier this week, with royal aides saying she had to find a balance in how she returned to public events. In January, the princess revealed she was in remission after her cancer diagnosis last year.
On Friday, she sent out a message about her support for children’s hospices – saying they helped families who were “heartbroken, fearful of the future and often desperately isolated”.
And now her photo has marked her husband’s birthday.
While Prince William was born in mid-summer on the longest day of the year, his father King Charles has been praising those in Antarctica experiencing the shortest day of the year.
He recorded a special message for the BBC’s Antarctic Midwinter Broadcast, which sends a morale-raising message to scientists working in remote research stations in the depths of their winter.
The King praised the work of researchers tracking climate change.