Black and blue
Colors tend to be charged with political and religious symbolism in northern Ireland. The "black north" that Stephen thinks of is an expression applied to the Protestant character of the whole region, and "true blue" evokes the Scottish settlers of the 17th century.
Thornton cites a saying in John J. Marshall's Popular Rhymes and Sayings of Ireland: "The Northern and Southern portions of Ireland have in later days been characterized as 'the Black North,' and the 'Sunny South.'" But the association may have begun in the late 1790s with the founding of a Protestant fraternal organization called The Royal Black Institution, two years after the founding of the Orange Order.
Gifford identifies "true blue" as a phrase used to describe "a seventeenth-century Scottish Prebyterian or Covenanter (from the color blue they adopted in opposition to the Royalists' red during the English Civil War)." Many of these fundamentalist Scottish Protestants colonized Ulster during James I's extension of the plantation system to that province.
In addition to black, blue, and orange, Ulster Protestants also claim the color purple, because a purple star was the symbol of the Williamite forces. Another Protestant fraternal organization related to the Orange Order is called the Royal Arch Purple.