Just like in Advent, there is a time in Lent when some choose to have pink (or rose) vestments instead of the normal Lenten purple. This is called Laetare Sunday or Rose Sunday, which is celebrated by some on the Fourth Sunday in Lent. The idea in the slight change in color is that there should be a lessening of our fast in Lent as a reminder of the joy we have in the Resurrection through Jesus Christ our Lord.
Thinking we need a lessening of our fast on one particular Sunday in Lent is a complete misunderstanding of Sundays in Lent, as well as in general. If we count every day including Sundays from Ash Wednesday to Holy Saturday, we get 46 days. Lent is 40 days. Those 6 extra days are Sundays.
Every Sunday is a feast to our Lord, never a fast. While Lent is a time for fasting, each Sunday in Lent is a feast, or a break, in the midst of our fasting. Every Sunday is a reminder of Jesus' death and Resurrection and a reminder of the joy we have in Jesus. Every Sunday is a chance for us to rejoice in the gift of God's grace.
Rose Sunday is, at best, redundant. We don't need a special reminder that even in our fasting we should remember the joy we have in Christ Jesus because we have that reminder every single Sunday. Every single Sunday in Lent is a break from our fast.
That means every Sunday you can indulge in whatever it is you are fasting from in Lent. It also means that just as in Advent, perhaps we shouldn't break up our seasonal unity in our liturgical colors to make a point that has already been made.
Further Reading:
Alexander, Neil. Celebrating Liturgical Time: Days, Weeks, and Seasons. New York: Church Publishing, 2014. pgs. 47-48.