Blue was the colour originally associated with Saint Patrick.
The association is linked to “St. Patrick’s blue,” a sky-blue shade adopted in the 1780s by the Anglo-Irish Order of St Patrick, a chivalric order founded in 1783. Blue also appears in older heraldic and royal imagery connected with Ireland.
Green became the dominant St Patrick’s Day colour much later. It grew from Irish nationalism, the shamrock tradition, and the symbolism of Ireland as the “Emerald Isle.” Green ribbons and shamrocks were being worn on the holiday by at least the 1680s, but the modern mass association developed over subsequent centuries.
A common mix-up is assuming that green was always Saint Patrick’s official colour. In fact, there has never been a single universally designated national colour of Ireland, although green is now the familiar cultural and festive choice.