Why are purple vestments used during Advent and Lent?

In the Roman Catholic Church, purple (or violet) vestments are worn during Advent and Lent as a symbol of penitence.12

Historically, the use of these colors is rooted in the liturgical developments formalized by Pope Innocent III, who utilized allegorical interpretations of scripture to assign colors to specific seasons. While black was traditionally used as a symbol of mourning on days of fasting and penance, violet was permitted as a mitigation of black during these seasons.2

Today, these periods are observed as times of preparation and reflection, and the use of purple reflects this penitential character. Exceptions occur on the third Sunday of Advent (Gaudete Sunday) and the fourth Sunday of Lent, when rose-colored vestments may be worn to signify a brief moment of joy and hope amidst the penitential season.132