Before the 2012 Olympics started the Royal Mail committed to turn a red post box gold for each British Gold medal, Olympic and Paralympic. In the event this meant that over 100 boxes turned colour. Each one is associated with a particular gold medal and has been chosen to be close to the medallist's home town/borough. Here's the map. We don't believe that the Royal Mail despatched an employee with a pot of gold paint the instant the medallist crossed the line but they have said the colour is permanent.
The Royal Mail have their own website of memorials, nationwide: https://www.royalmail.com/memorials/home .
2022: Londonist have posted (ha ha) an excellent page on postboxes.
2024: We read a WW2 'Letter from London' by Mollie Panter-Downes with: "Posting a letter has acquired a new interest, too, since His Majesty's tubby scarlet pillar boxes have been done up in squares of yellow detector paint, which changes colour if there is poison gas in the air and is said to be as sensitive as a chameleon."
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