An early use of red at Christmas were the apples on the paradise tree. They represented the fall of Adam. Red is also the color of Holly berries, which is said to represent the blood of Jesus when he died on the cross. Red is also the color of Bishops robes.
In 2004, Pope John Paul called the Christmas tree a symbol of Christ. This very ancient custom, he said, exalts the value of life, as in winter what is evergreen becomes a sign of undying life, and it reminds Christians of the "tree of life" of Genesis 2:9, an image of Christ, the supreme gift of God to humanity.
But, to some grinches here politifact knows best. The Red Tree theme was challenged by politifact because, well, Mrs. Trump made the selection.
By the way, in 2016 Christmas trees were tinted every color imaginable, including red, and were all the rage:
https://www.today.com/home/colorful-real-christmas-trees-t105764But, that was before Mrs. Trump made the selection and politifact made the challenge...because, well, Mrs. Trump made the selection.