A.Post (or pillar) boxes have been red since their introduction in the UK in 1855, so they can be easily identified. Smaller post boxes were black.
Q.Why do post boxes say ER on them
A.The ER on Royal Mail vans and post boxes stands for Elizabeth Regina, Elizabeth the Queen. You will see this on most products with a royal endorsement.
Q.What colour are French post boxes
A.Most French Letter boxes are coloured yellow like the sign for the French Post Office, La Poste, which is yellow with blue writing.