We’ve been asked why we use four purple candles in the Advent wreath instead of three purple candles and one rose. That’s a good question. The following explanation is taken from The United Methodist Book of Worship which serves as a source of authority in matters of worship.
The Advent wreath is a simple circle of evergreen branches, a sign of life without end; its four Advent candles encircle a central white Christ candle. Some traditions use the color rose on the Third Sunday of Advent and for this reason use three purple candles and one rose candle. United Methodists, however, encourage purple for the whole season of Advent and therefore use four purple candles. 
 United Methodist Church, The United Methodist Book of Worship (Nashville, TN: United Methodist Publishing House, 1992), 261.