The Royal Company of Archers and their white and green cockades (not Hanoverian black) in their bonnets, it is indeed an interesting story and one with strong Jacobite connections. The official line is that the company was formed in 1676 to be the King’s body guard in Scotland, though some claim their origins date back to James VI. Then, as now, it was an elite group of men.
They adopted the tartan uniform – which is often seen in discussions about late 17th and early 18th century tartans. They were led by a number of well known Jacobites, including James Fifth Earl of Wemyss, at the time of the ’45. After the ’45, they were regarded with great suspicion, so when George IV famously came to Edinburgh in 1822, the company adopted a uniform (still bearing odd throw-backs to previous centuries) of the Government tartan, also known as the Black Watch.
As the historian for the company wrote in 1875: ‘The spirits of the old Jacobite members might well have stood aghast, if they could have beheld their successors guarding into Edinburgh the carriage of a King of the house of Brunswick [ie Hanover].’
From the middle of the 19th century, the current uniform was adopted and worn, fairly unchanged ever since. It features a hardened bonnet (similar to those worn in the Highland Revival) in dark green, the feather, and a white and green cockade.
Ref: the National War Museum, the Royal Company of Archers.