what does the red mean?

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The Queen’s Guard are responsible for guarding Buckingham Palace and St. James’s Palace in London. They usually consist of Foot Guards  wearing full-dress uniform of red tunics and bearskins hats.
Altogether the Guard consists of three officers and 36 soldiers.
From a distance the full dress uniforms worn by the men of the five Regiments of Foot Guards look identical, but look  closely and you will spot the differences!