In the armies of the 1600s the Colonel of each regiment provided the uniform for his men, and so regiments wore coats of different colours. By the late seventeenth century, most English regiments wore red or crimson coats, but some wore blue or grey. Regiments in foreign armies, too, wore coats of various colours. The English Army (which became the British Army following the Act of Union with Scotland in 1707) often fought alongside Danish, Hanoverian, Prussian, etc, troops, all with different uniforms. Therefore it was necessary, when going into battle, to display a 'field sign' in their hats, to distinguish their own side from the enemy. There are numerous contemporary references in soldiers' memoirs to the issue of a sprig of oak leaves, or a piece of white paper, or a bunch of ribbons worn in the hat to indicate which soldiers were on their side. The bunch of ribbons was tidied up and became known as a cockade, and worn in the hat or cap it became an significant part of a soldier's uniform throughout the 18th and 19th centuries. Soldiers of the Stuart kings wore a white cockade. Following the overthrow of King James II of England in 1688, his followers ('Jacobites')continued to wear the white cockade. Soldiers of the French Royalist Army also wore a white cockade, until the French Revolution (1789), when it was replaced by a cockade of blue, white and red. From early in the 18th century, (there is evidence in a picture c1712) British soldiers have worn a black cockade in their hats or caps; it is still in evidence in certain forms of modern British Army uniform, eg on the glengarry headgear of Scottish regiments. Other national armies wore cockades of other colours. At the Battle of Culloden (1746) the Jacobites wore white cockades in their bonnets, while the British redcoats of King George II who were opposing them wore black cockades.
In the song, as the man is recruited into the British Army, the lyrics ought really to refer to a black cockade!