Many Lutheran congregations and pastors are using blue paraments and vestments during Advent. However, this was an innovation introduced in the last 30 years. The traditional color for Advent is purple, and in the Roman Catholic Church, the use of blue has never been officially approved for use in Advent. Why then the switch to blue? Today's liturgists tell us that the old penitential emphasis of Advent has been replaced with an emphasis on eschatological hope in the future kingdom of God (or "reign of God" for those who don't like the maleness of the word kingdom). Blue represents royalty (of course, so does purple!) and hope, so they say. These are the same liturgists who have said that we should lose the penitential emphasis in Lent and move to more of a stress on baptism and catechesis. Yeah, whatever. It seems the [post-] modern liturgists have a problem with penitence. I guess this is why the new ELW allows a renewal of baptismal vows to replace the order for confession and forgiveness. I don't have a problem with an order for renewal of baptismal vows with sprinkling from the font, but this should be done as an amplification of the Creed (Easter, All Saints, and the Baptism of our Lord would be good feast days for this). However, I am not for replacing confession and forgiveness with a renewal of baptismal vows. We need regular confession of sins more than ever in our churches.
In any case, back to blue in Advent. It must be admitted that the use of blue in Advent does have some historical basis. In the middle ages, a dark blue (indigo) was in use in England and in Scandinavia where it was more difficult to obtain the ingredients for purple dye. Blue is also used in certain churches is Spain and Latin America, but here it is a lighter blue connected with feasts of the Virgin Mary. Certainly, if blue is used for Advent it should be of the darker indigo variety so that some of the penitential character of the season remains. After all, the new Revised Common Lectionary does focus on John the Baptist's call to repentance. In my own church, we use a Sarum blue. This is something I inherited. When Lutherans spend money on something, they don't like to be told it wasn't right. Anyway, I can live with it, though I prefer the traditional purple.
With regard to the color rose: unfortunately, the LBW altar edition/minister's desk edition gives the colors purple or blue for use on the third Sunday in Advent. LBW makes no mention of the option of using rose vestments and paraments. However, the rose color has a long history in the Church and many churches have a rose-colored candle for the third week in Advent on the Advent wreath. The rose vestments and paraments being brighter than the penitent purple correspond to the ancient introit (entrance antiphon) from which this Sunday--Gaudete Sunday-- gets its name: Gaudete in Domino semper ("Rejoice in the Lord always") from the 4th Chapter of Saint Paul's Letter to the Philippians. On this Sunday there was a reprieve from the strict fasting of Advent when Advent was truly a penitential season. A similar reprieve took place in Lent on the 4th Sunday, the introit of which also begins with "Rejoice (Laetare) O Jerusalem!," thus it is known as Laetare Sunday. It is perfectly acceptable and commendable for Lutherans to make use of rose vestments and paraments on both Gaudete and Laetare Sundays.
Note: It is not appropriate to light the rose candle on the Advent wreath on the 4th Sunday of Advent because "Christmas is almost here." That's a silly innovation. If we are going to reclaim liturgical ceremonies and practices, let us at least use them properly and at the right times.