The original color associated with St. Patrick is blue, not green as commonly believed. In several works of art depicting the saint, he is shown wearing blue vestments. Green was associated with Ireland after St. Patrick's life, possibly because of the greenness of the Irish countryside.

Also, St. Patrick's blue is a name applied to several shades of blue associated with St. Patrick and Ireland. Blue as a color was initially associated with St. Patrick when it was adopted as the color of the Anglo-Irish "Order of St. Patrick" around the 1780s. In British usage, it refers to a sky blue used by the Order of St. Patrick, whereas in Irish usage it is often used with dark, rich blue symbols (items).

Today, St. Patrick's blue is still found on key symbols used by both the state and the island of Ireland.

More Info: en.wikipedia.org