The Third Sunday of Advent is called Gaudete Sunday! Rejoice!
We light the Rose Candle on this day (not "pink"), to rejoice in the Lord, ALWAYS! We are joyful not only for the coming of the ChristMass and celebrating the birth of our Lord and Savior, Jesus Christ, but also for His glorious victory over sin and death celebrated not only on Easter Sunday, but EVERY Sunday which is like a "little Easter" every week! Through the Mass we remember His life, death, and resurrection every single time we celebrate the Mass. Likewise, every Friday is like a "little Good Friday," a time to reflect upon the unequaled Sacrifice Jesus Christ made for us - which is why we still observe some sort of penance (or act of mercy) which used to be the abstinence of meat on Fridays (and is still recommended to be that or equivalent to that) based on Canon 1250-1252, still in force and effect, of Canon Law, but I digress.
The Rose Candle is remembrance of the joy in our hearts for the risen Lord - afterall, He didn't "pink" for us on Easter Sunday, He "rose" for us!