Q: Is indigo a required color for Advent to distinguish it from Lent? And is rose required for the Third Sunday of Advent and Fourth Sunday of Lent?
A: The GIRM uses the same names for the color of the vestments in Advent and Lent: “violet or purple” (346d). Some like to distinguish the two seasons by wearing different shades, but that is not required.
The color rose “may be used, with it is the practice” on those two Sundays (346f).