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.