I think it's a play off of the color Purple. Red would lend itself to being a more "aggressive" color, and the twist of it symbolizing the blood of a martyr could easily be added.....
(still looking)
Historically little boys were often dressed in red or pink (seen as variants of the same colour) because it was considered vital and giving energy. Opposite, small girls were often dressed in blue, because it was a calmer and more "inactive" colour, which of course was desirable in a girl. There is a bit about this in the book "Blue, the history of a color", by Michel Pastoreau.
"Amilcare Anguissola with the children Minerva and Asdrubale", ca. 1560s, by Sofonisba Anguissola
Amusingly enough, people think it's borderlining child abuse to dress a baby boy in pink and a baby girl in blue today. The colours are used opposite than before to mark the gender of the child. Whether the idea of the qualities of the colours are the same is up for interpretation, but it's interesting to think we today want to give the girls more energy, while we want to calm the boys...
The Virgin Mary and Jesus is two figures often seen wearing both colours in period depictions. There the idea of colour was that red represented the passion and the blood spilt, while the blue represented the eternal sky (and indirectly God's kingdom). The idea of red being vital and passionate is most likely related to the colour of blood.
In mundane clothing red usually wasn't reserved for one specific class. Everyone could wear it. However, some dyes used to create red were for nobility only. This was because they provided richer red shades which lasted longer, and hence were a lot more expensive to make. But cheaper red colours were usually free to wear for everyone. Interesting to see that different rules existed within brotherhoods.
