"In our Advent Wreath, three candles are purple and one is pink. Why the one pink candle?"
Before I answer the specific question, for those who aren't familiar with the church "liturgical" year, let's look at the season of Advent.
Advent is the beginning of the church year. The first Sunday of Advent (and the first Sunday of the church year) is the fourth Sunday preceding Christmas day. This year it was November 30th. The church divides the year into seasons, much like our meteorological year. The seasons are Advent, Christmastide, Epiphany, Lent, Eastertide, and finally Pentecost. Although there has been a move in some denominations to remove the seasons of Epiphany and Pentecost and rename them Ordinary Time, my personal belief is there is no "ordinary time" in the church. All seasons in the church are extraordinary.
In any event, Advent is a four-Sunday season of preparation in the church (and the world) for the arrival of Christ into the world, both at Christmas and in our lives. An Advent wreath is one of the traditional accessories for the season. The wreath is decorated with greenery and laid upon a stand. A large white candle, called the Christ Candle, stands in the middle of the wreath and four candles are placed into the wreath surrounding the central candle. The four candles may be all blue, all purple, or three purple and one pink. These candles are lit on successive Sundays in Advent, one for each week. The pink one is lit on the third week and the Christ Candle is lit on either Christmas Eve or Christmas day, and on each Sunday during Christmastide.
Now, why the pink candle? In the earliest years of the church the only church season was Lent, the seven weeks prior to Easter. Lent was a season of fasting and prayer as the church commemorated the crucifixion of Jesus. The traditional color of banners in the church during this time was a deep purple, signifying royalty, repentance, and suffering. During Lent the church lit seven candles, one for each week of the solemn season. However solemn the season, the story of Lent also has a twinge of hope and joy since the death of Christ prefigured the resurrection. So, on the third Sunday of Lent, the church was encouraged not to fast, but to feast. In ancient times on this particular Sunday the Pope would honor a citizen with a pink rose, and as time passed the priests wore pink vestments on this day as a reminder of the coming joy.
When the season of Advent was instituted the church viewed it as a mini-Lent, a time for reflection and repentance (thus the purple). In so doing, the church adopted the first four candles of Lent and changed the third candle of Advent to pink in honor of the Lenten tradition. This is why we have a pink candle in our Advent Wreaths.
To further heighten the sense of anticipation of Christ's coming during Advent, the church named each candle in the wreath -- the first being hope, the second peace, the third joy, and the fourth love (there are a number of other traditional names as well, though these are some of the most ancient). It has always seemed fitting to me that the pink candle is the candle of joy, the one that speaks to us with its twinge of color.