In some circles, the 3rd Sunday of Advent is also known as "Gaudate Sunday". Gaudate is Latin for "rejoice", taken from the first word of the reading from Philippians, the reading for the Sunday back in the Middle Ages as well as our reading during Year C of the Revised Common Lectionary.
Those words led to a desire in the Middle Ages to "lessen" the sense of penitence in Advent on this Sunday. Thus began the use of pink on this Sunday (you can learn more about the use of Pink for the Advent wreath and vestments by clicking here), itself a further fading of the colors that resulted from trying to create black dyes for Advent and Lent (you can learn more about the use of either blue or purple for Advent by clicking here).
We should note that there is no need to lessen the penitential sense for either Advent or Lent on a Sunday. Sundays are always feasts in the church, even when they come in the middle of penitential seasons or fasts.