Catholic Nun at Canonization - Catholic Stock Photo

In the Roman Catholic tradition, there are many religious institutes of nuns and sisters (the female equivalent of male monks or friars), each with its own charism or special character. Traditionally, nuns are members of enclosed religious orders and take solemn religious vows, while sisters do not live in the papal enclosure and formerly took vows called "simple vows". Several orders have adopted habits that are light blue either partially (such as a light blue veil) or completely. Sky blue is the traditional color of the Virgin Mary, and orders who wear this color are displaying a special devotion to her.

Source