The Scots Guards have their roots in ‘His Majesty’s Foot Guards of Scotland‘ raised in 1639, who saw much service in Ireland. They came on to the establishment as the 3rd Guards in 1661, and acquired their present title, Scots Guards, in 1877. In 1815 the 1st Guards received as a battle honour the title Grenadier Guards. In 1831 it was decided to designate the Coldstream and the 3rd Guards as fusilier guards, and to dress them as fusiliers, whose dress was by then virtually the same as that of grenadiers. So, in that year both guards regiments were dressed in bearskin caps and epaulettes. The 3rd Guards were renamed the Scots Fusilier Guards, but the title of the Coldstream Guards was not altered. This is the uniform of a field officer of the regiment immediately after the change. The gold cords shown on the bearskin cap of 1821 have gone, but the tassels remain, and there is a regimental badge of a crown over a thistle in front of the cap. The white grenadier plume shown here was worn for only a short time, and afterwards the cap was worn with no plume, as today. The tassels and badge disappeared at the same time. The sword is carried in a frog mounted on a waist-belt, which is unusual. The gold and crimson sash and the gold-striped trousers indicate that the wearer is in court dress, or guard of honour order.
Source: Portrait at Windsor Castle of Lord Rokeby by Alexandre-Jean Dubois-Drahonet.