Q: Why is Mary wearing red?
A: There is a tradition which holds that the use of red for Mary's robe derives from the belief that Mary, as a child in the Temple, was involved in weaving the great temple veil (that would be rent at the time of Jesus' death). In particular, it was Mary's task to weave in and out the crimson threads. It's an interesting concept--since the temple veil concealed the Holy of Holies--and Mary's garment conceals her body which carried within it, God, Himself.
This explanation is based on the Proto-Evangelium of James. Mary's spinning in the Temple is mentioned in chapter 10.2: "And to Mary fell the lot of the 'pure purple' and scarlet." The reference to purple is mentioned several times in Exodus (e.g. Ex 35:25). The confusing part is about the variety of colors mentioned for the "veil of the temple": gold, amiant, linen, silk, the hyazinth-blue ... thus a multicolored veil of which Mary spun a fraction. This may be a reason why the interpretation based on the Proto-Evangelium was not more readily followed. The most common one is that of red designating the loving mother and physical person of Mary.