My understanding - which I will have to re-check given your post - is that the more traditional representation is that the dress is marian blue, and the mantle generally depicted as red.
It's interesting that these are also, as you mention, the two predominant colours used on both the Papess and the Empress.
Personally, and as I have also mentioned in the threads on both these cards, I tend to see the first (II Papess) as having close iconographic connection to the annunciation, and the second (III Empress) as very much having iconographic connections to Mary & Child (and thus/also Isis & Horus).