Historically, the British Armed Forces fought in red, a traditional colour that remains in use in formal and ceremonial uniforms . Militarily, and in other contexts, the single chief colour associated with the UK is therefore red, such as in the famous British Army red coats [Barnes, Major R. M. (1951) History of the Regiments & Uniforms of the British Army]. Additionally, at sea, the British were identified by the Red Ensign - a flag that is almost entirely red.

However, red is a difficult colour to overprint on a map and still allow the black lettering to be visible. So, printers compromised by choosing pink for Britain and its empire. [Painting the World Pink - https://manchesterhistorian.com/2013/painting-the-world-pink]