I'm also accepting just "George" as the answer here.

Public post boxes in the UK have normally carried the initials, or cypher, of the reigning monarch. These consist of the first character of the regnal name (e.g. George), "R" for either Rex (king) or Regina (Queen), and the monarch's regnal number (usually). [What a pity that the initial R does not indicate the monarch's gender]. The design has changed over the years, and between 1879 and 1892 the design omitted the royal cypher and the words "Post Office"; these boxes are referred to as "anonymous" boxes as a result. George V boxes and E R II (Elizabeth the Second) are the most common, but you may find boxes with any of the six monarchs since the postal service was setup:

• VR – Queen Victoria (postbox put up between 1853-1901)
• ER VII – Edward VII (postbox put up between 1901-1910)
• GR – George V (postbox put up between 1910-1936)
• ER VIII – Edward VIII (postbox put up between 20 January 1936 – 11 December 1936)
• GR VI – George VI (postbox put up between 1936-1952)
• ER II – Elizabeth II (postbox put up between 1952-current)

George V postboxes are the only ones not to carry the regnal number, where the monarch's regnal number is greater than 1. I'm assuming this is because, in 1910 when George V was crowned, there had been no previous "G"-initialled monarch reigning since post boxes were introduced. When Charles takes the throne, he will most likely (and confusingly) be George VII; so it won't be until his son William succeeds him that we'll prove that theory (if he chooses the name King William, he will be William V)

So what colour should a pillar box be? Well, the obvious answer is pillar-box red; but if you're in Guernsey, you'll be familiar with blue pillar boxes; if in Cyprus, with yellow; and of course after the 2012 London Olympics, selected pillar boxes throughout the UK were painted gold in the home towns of gold medal winners - some remain in this colour.