Soldiers of this regiment were distinguishable by the unique feature of the "flash", consisting of five overlapping black silk ribbons (seven inches long for soldiers and nine inches long for officers) on the back of the uniform jacket at neck level. This is a legacy of the days when it was normal for soldiers to wear pigtails. In 1808, this practice was discontinued, but the RWF were serving in Nova Scotia when the order to discontinue the use of the flash was issued. Upon their return they decided to retain the ribbons with which the pigtail was tied, and were granted this special concession by the King. The Army Council attempted to remove the flash during the First World War citing the grounds that it would help the Germans identify which unit was facing them. As Fusilier Robert Graves reported, "the regiment retorted by inquiring on what occasion since the retreat from Corunna, when the regiment was the last to leave Spain, with the keys of the town postern in the pocket of one of its officers, had any of His Majesty's enemies seen the back of a Royal Welch Fusilier?," and the matter remained "in abeyance throughout the war."During the Napoleonic Wars, they served from 1810 to 1814 in the Peninsular War; fighting at Albuera, Badajoz, Salamanca, the Pyrenees, Nivelle and Toulouse and took part in theBattle of Waterloo. where they fought in the 4th Brigade under Lt-Col. Hugh Henry Mitchell, in the 4th British Infantry Division (see Order of Battle of the Waterloo Campaign.
As a fusilier regiment, the RWF wore a hackle, which consisted of a plume of white feathers mounted behind the cap-badge of the modern beret. The full dress of the Royal Welch Fusiliers, as worn by the entire regiment until 1914, included a racoon-skin hat (bearskin for officers) with a white hackle and a scarlet tunic with the dark blue facings of a Royal regiment. This uniform continued to be worn by the RWF's Corps of Drums and the Regimental Pioneers until the amalgamation of 2006.