"On 4 December 1774, the HMS Asia, Somerset, and Boynes, carrying 460 British Marines under the command of the legendary Major John Pitcairn, arrived upon the shores of the tumultous city of Boston. A month later, sensing New England's bellicosity, the Admiralty readied additional forces for service in the colonies. In May of 1775, this augmentation, consisting of 2 majors, 10 captains, 27 subalterns, 28 sergeants, 25 corporals, 29 drummers, and 600 privates, joined with the 460 marines already in Boston to form the 1st and 2nd Battalion of British Marines.
Marines of the 18th c were generally not regimented, but rather worked in small units aboard vessels and ashore. Therefore, colours were not as intregal to the marines as with army regiments. The exception to this, of course, is the two battalions of marines stationed in Boston in 1775. There were three grand divisions of marines in Portsmouth, Plymouth and Chatham, England. All three of these may have had different colours, or not.
In 1760 a merchant named William Nicholson submitted a bill for one Union sheet of Colours of silk with the embroided Arms of the Lord High Admiral surrounded by thistles and roses. The Arms of the Lord High Admiral is the fouled anchor. A description from 1770 tells of a completely different marine colour: the Union throughout and a ship with furled sails in the center.
The primary source of this information comes from 18th c. letters and orders compiled in a paper by General HE Blumburg in the 1930s. This information is further echoed in For the Glory of the Marines! by Thomas Boaz."