First, it’s important to define the exact color in question. The liturgical color that can be worn on the third Sunday of Advent (“Gaudete Sunday”) is rosacea, or “rose” – not pink. Rosacea has a slight orangish-red tint to it, sort of like a fresh salmon filet. If you’ve never actually seen rose-colored vestments before, you might be surprised to find that they are very distinguished and beautiful. Pink, on the other hand, is a paler, more feminine color. Think “Pepto-Bismol”.
In our modern times, vestments for this day have gotten a little bubble-gum crazy and, as a result, a priest risks looking like a big Care Bear every time Gaudete Sunday rolls around. Thankfully, he can also choose to simply wear violet.
Here are some examples of rose-colored vestments, courtesy of Fr. Z and New Liturgical Movement (go to each site and search for "Gaudete" and "rosacea"):
As you can see, there is some acceptable variation in color. Some of the vestments above are closer to red, others closer to purple. All of them are very dignified and appropriate for the third Sunday of Advent.
The reason for the color change is to emphasize in a poignant way that the Lord is near. Advent is now more than half-way over! The bursting forth of such an unusual color has the effect of a sudden exclamation in a quiet room. In the midst of our penances, and our quiet contemplation, a voice cries out: Gaudete in Domino semper! “Rejoice in the Lord always!” Those are the words of the Entrance Antiphon for today, and that’s why we call this day “Gaudete Sunday.”
Where does the Advent wreath come from? What does it mean?
The following explanation is from Fr. William Saunders, a popular Catholic apologist and theologian:
The Advent wreath is part of our long-standing Catholic tradition. However, the actual origins are uncertain. There is evidence of pre-Christian Germanic peoples using wreaths with lit candles during the cold and dark December days as a sign of hope in the future warm and extended-sunlight days of spring. In Scandinavia during winter, lit candles were placed around a wheel, and prayers were offered to the god of light to turn "the wheel of the earth" back toward the sun to lengthen the days and restore warmth.
By the Middle Ages, Christians had taken up this practice and infused it with profound symbolism as a way to prepare for Christmas. Since Jesus is the light of the world (cf. Jn 8:12), it is fitting that the wreath would produce more and more light the closer we get to Christmas, the celebration of the birth of Christ. The four candles represent the four weeks of Advent. Three of them are purple, a color traditionally associated with penance and prayer. One of them is rose, a symbol of our rejoicing. The circular shape of the wreath, which has no beginning or end, symbolizes the eternity of God, the immortality of the soul and the everlasting life found in Christ.