Let 's solve the philatelic riddle: for years it was red and turned purple. Have you guessed it? Yes, it is the classic everyday 63p stamp, which features the Queen Elizabeth's profile by sculptor Arnold Machin that has switched from its regular red to regal purple. Royal Mail decided to change the color of this stamp to underline the importance of longest reigning Queen – Elizabeth II. Its iridescent overprint, which usually reads "Royal Mail", now bears the text "Long to Reign Over Us", Royal Mail revealed. It is the first time the colour purple has ever been used for what is known as the First Class Definitive stamp. Andrew Hammond, stamps and collectibles director at Royal Mail, said: "Royal Mail stamps mark the most significant anniversaries and landmark events in the life of our country. We are proud to commemorate Her Majesty becoming the longest-reigning monarch in UK history".