Purple is a penitential colour of fasting while pink (rose) is the colour of joy.
The 3rd Sunday in Advent is Gaudete Sunday (from the Latin meaning “rejoice”) which is taken from Philippians 4:4-5, the Entrance Antiphon of the day. Long ago the Pope would honour a citizen with a pink rose (or a rose rose?) Priests then would wear pink vestments as a reminder of this coming joy. Rose is also used during Laetare Sunday (the Fourth Sunday of Lent) to symbolize a similar expectation of the coming joy of Christ’s coming in Easter.
Other Resources:
http://www.newadvent.org/cathen/06394b.htm
https://en.wikipedia.org/wiki/Gaudete_Sunday
