Why red and what type of red when it comes to the garments of St Mary Magdalene?
To highlight theelement of wealth and nobility* in the woman which, when freed from evil spirits , followed and supported materially Jesus throughout His teaching along with other women (named Holy Myrrh-bearers in the Orthodox Church), risked and gave everything for Him and rewarded by being the first to Witness His resurrection.
And I believe that is the same element we highlight by painting martyrs in red- The element of nobility, acquired not by heritage but by their virtues and their suffering.