Royal Mail celebrated every gold medal won by a British athlete during the London 2012 Olympic Games by repainting one of their iconic red pillar boxes gold for each gold medal winning Olympian. This one honours Geraint Thomas who won a gold medal in the men's cycling team pursuit event. It is on Castle Street, opposite Cardiff Castle.
In total, Team GB won 29 Gold Medals for the Olympic Games and 34 Gold Medals for the Paralympic Games; for team medals, a post box was painted gold for all team members (see
Link for list and more examples).
This was the first occasion in modern times that Royal Mail had changed the colour of their post boxes from the traditional red which has been the standard colour for UK boxes from 1874, with very few exceptions. The original plan was for the boxes to be returned to their traditional red colour but in November, it was announced that they would remain gold and that a plaque would be fixed to each one (
Link BBC News).
Archive Link - archived 31st December 2017 (Royal Mail Gold Postbox Finder)