Did you know that before 1955 (liturgical reforms) the colour for the entire Palm Sunday liturgy was purple? Purple vestments were used for both the Procession and the Mass. This was because, and as it still is, Palm Sunday and Holy Week are part of the Lenten penitential period. Thus, purple was used just like the rest of the days of the Lenten Season.
In 1955 this was changed to red for the Procession, while purple was maintained for the Mass. Here, the Procession was understood as linked to the Passion of our Lord hence, the change to red.
In 1969, following the Second Vatican Council reforms, red became the colour for both the Procession and the Mass. The focus of the entire Palm Sunday service was shifted to the Passion of our Lord so the red colour now became the only colour used for the day. It was also at this time that the official name of the day was changed to Palm Sunday of the Passion of Our Lord. Before, it was simply called Palm Sunday, a title which is still popular even today.
Holy Mass: Purple
Holy Mass: Purple
Holy Mass: Red
PS: In Catholic liturgical art, red stands for the Divine (divinity) and blue, for the human (humanity). This explains why Jesus is usually pictured clothed with an inner red dress and an outer one of blue depicting the divinity and humanity of Jesus (eg. Image of the Sacred Heart). He is first divine (red) but then took on our humanity (blue) through his Incarnation. In the case of Our Lady, the inverse is usually the case. She is usually shown in a blue inner dress with a red outer. She is human (blue) but God clothed her with divinity (red) when she accepted to become the Mother of God. This is the journey of all of us humans; our hope is that one day God will clothe us also with his divine nature.
Once again, we see how Mother Church through her use of colours gives us a theology in silence. The more we know and understand these signs, the better we appreciate our rich Catholic worship.