United Kingdom military units usually carry two Regulation Colours. These are the Regulation Queen's Colour and Regulation Regimental Colour. These are often referred to as the Standard or Ensign.
Colours are the identifying battle flags carried by military regiments to show where their respective troops should rally to in battle. Originally these were 6'6" x 6" in size, though have now been reduced to 3'9" x 3', as regiments no longer carry their colours on the battlefield.
Prior to 1743, each regiment was responsible for the design and quantity of standards carried, often with each company having its own design. In that year King George II issued a royal warrant to require each regiment to have, as their first colour, the King's colour. It was also to consist of the Union flag throughout, as a symbol of royal allegiance. The second colour was to be the colour of the facings (uniform linings) of each Regiment, with the Union flag in the upper canton. When Regiments had red or white facings, this was to be replaced by the red cross of St George on a white background.
A second royal warrant was issued in 1747, requiring the Regimental number to be displayed on the colours. As many regiments at that time were known by the name of the Regimental Colonel instead of a number, this requirement was often ignored. On July 1, 1751 a third royal warrant was issued stating "No Colonel is to put his Arms, Crest, Device, or Livery on any part of the Appointments of the Regiment under his Command."
Over time, these colours have evolved to include the battle honors awarded to each Regiment, though these have also been limited.
Only one British Regiment carries more than two colours on parade. This is the Duke of Wellington's Regiment (West Riding). The 'Dukes', uniquely, carry four colours. The second pair consists of a stand of honorary battle flags, which are the original size of 6'6" x 6'. These honorary colours, Queen's honorary colour and regimental honorary colour, were awarded to the 76th Regiment of Foot, which later to become the 2nd Battalion of the Duke of Wellington's Regiment in 1808, by the Honorable East India Company for their distinguished services during the Battle of Ally Ghur and again at Delhi between 1802 and 1804.
The original article can be found at Regulation Colours and the edit history here.