The fourth Sunday of Lent is rather unique; like the third Sunday of Advent ("Gaudete Sunday"), the fourth Sunday of Lent is a break in an otherwise penitential season. The vestments for this day will be rose, as they are on Gaudete Sunday in Advent, and flowers may adorn the Altar. This day is called "Laetare Sunday" (also "Rose Sunday" ), and takes its name from the opening words of the Mass, the Introit's "Laetare, Jerusalem"...It really did lighten my mood this morning to see the rose colored vestments and stop to think about the joy that awaits us at Easter ... and that can be found in the midst of this penitential season.
A bit more info comes from Fr. Dwight Longenecker:
The Rose color was made from the very rare crimson dye taken from a tiny gland in the murex mollusc (a kind of sea snail) found only off the coast of Lebanon. Thus, in the ancient world that particular rose color was a sign of great wealth, and royal status. The High Priest in the temple in Jerusalem used it in his vestments. It came to be used on the two refreshment Sundays in the penitential seasons to perk people up.
But there is more to it than that. The rose vestments in Lent, remind us of the royal and priestly status of Our Lord. The priest in persona Christi presents an icon of Christ the King and great High Priest. The fact that this image is stuck in the midst of the two penitential seasons reminds us that locked into the austerity of this world, robed in the squalor and simplicity of human flesh, there lies hidden the Great High Priest of the New Covenant, Christ the King of Kings and Lord of Lords.