The National Covenant and The Solemn League and Covenant (a.k.a. Covenanters) signed documents stating that Scotland desired a Presbyterian Church government, and rejected the Church of England as their official church (no Anglican congregation was ever accepted as the official church in Scotland). What the Covenanters rejected was episcopacy — rule by bishops — the preferred form of church government in England. Many of the Covenanters signed these documents using their own blood, and many in the movement began wearing red pieces of cloth around their neck to signify their position to the public. They were referred to as rednecks.[1] Large numbers of these Scottish Presbyterians migrated from their lowland Scottish home to Ulster (the northern province of Ireland) and soon settled in considerable numbers in North America throughout the 18th century. Some emigrated directly from Scotland to the American colonies in the late 18th and early 19th-centuries as a result of the Lowland Clearances. This etymological theory holds that since many Scots-Irish Americans and Scottish Americans who settled in Appalachia and the South were Presbyterian, the term was bestowed upon them and their descendants.
Wikipedia
10-15K of them wore red bandannas around their necks to show which side they were on.