It seems to me that on the original the red horse was darker than Flornoy made it. I include the Cary Sheet because that's the first known card in that style.
I don't think one should compare the card with what some artist drew in a different century, illustrating a different part of the allegory. What was available in 1441 was the Plato, newly translated and causing a furor, because of homoerotic imagery (easily edited out). I will give a little of it. It is very complex, many parts. I hope I will give enough. If not, you can go to http://classics.mit.edu/Plato/phaedrus.html. It's an old translation, reliable in some parts and not others. I read several translations, as well as commentaries. Probably there is a better version in Italian. I can't imagine reading Plato in a foreign language!
Here is what Plato says, related to the Noblet image:
From scholarly discussions of the Phaedrus (not presently at hand), I understand that it is not exactly clear what the Greek word sometimes translated as "black" (*dark" here) actually means; it might be "dun." I remember once in Catalonia (Figueres, actually), in a restaurant I was trying to clarify what kind of wine I was ordering, and the waiter pointed to his black shoes while saying the word in Catalan. We got a red wine. So we have the two horses, one darker than the other. Since the darker one is hot-blooded, and even with a "blood-red complexion", it is natural to paint him red, which is more interesting than black and conveys his hot-bloodedness. Some Tarot de Marseille decks painted the two horses the same color. I assumed that they hadn't realized the significance of the colors.The right-hand horse is upright and cleanly made: he has a lofty neck and an aquiline nose: his colour is white, and his eyes dark; he is a lover of honour and modesty and temperance, and the follower of true glory; he needs no touch of the whip, but is guided by word and admonition only. The other is a crooked lumbering animal, put together anyhow; he has a short thick neck; he is flat-faced and of a dark colour, with grey eyes and blood-red complexion; the mate of insolence and pride, shag-eared and deaf, hardly yielding to whip and spur.
Jean-Michele also quoted the Republic, Book IV, with its three-part division of the soul, Reason as the controller, a Spirited part as the part that wants to do noble deeds and is ashamed of the ignoble, and Appetite as the rest, pertaining to the physical aspects of life. So Reason sits on top, and the two horses are the Noble and the Ignoble, i.e. Spirit and Appetite. The three together equal the soul. (The three parts of the soul also correspond to three of the cardinal virtues, which are floating around here somewhere, but that's another issue.)
In the Noblet there are no reins. The Charioteer, Reason, commands the noble horse with his voice. As for the ignoble one, well, that's a another part, which I'll get to next. The horses used to have wings, but they lost them. That's an earlier part of the story. I'll get to that later.
After the part I just quoted, the text gets difficult, and Jowett isn't the best translation, so I will paraphrase. What happens is that the three of them see the Beloved in the distance, who is not described. but has to be a person with a beautiful body, spirit, and mind. The unruly horse lunges forward and won't be dissuaded from following his lust. The charioteer and the noble horse actually yield temporarily to that horse, and then the charioteer, seeing him or her up close, remembers the archetypal Beauty he had seen among the gods when the horses had wings and he was still in the heavens. He and the noble horse are both filled with awe and respect, and they manage to force the ignoble horse to back off.. In this part there is a graphic description of how the charioteer has to pull so hard on the bridle that the unruly horse's mouth is filled with blood. (The ironic part is that the charioteer would never have remembered Beauty without the ignoble horse's lust.) Finally:
"Modesty" applies to the noble horse, "holy fear" to the ignoble one. That result, once the ignoble horse is tamed, is what is pictured on the Noblet, the ignoble horse following the lead of the noble horse with its head (even though the body wants to go elsewhere) and the noble horse listening to the words of the charioteer....when this has happened several times and the villain has ceased from his wanton way, he is tamed and humbled, and follows the will of the charioteer, and when he sees the beautiful one he is ready to die of fear. And from that time forward the soul of the lover follows the beloved in modesty and holy fear.
The Noblet really does the best job of illustrating the triumph of the charioteer, reason and spirit over appetite.
But as I looked at other historical versions of the card, I could see the same thing, more or less. In the Issy, which Ross says Thierry Depaulis dates to 1450, there are the same two colors, although the horses have riders and the woman, the beloved, is now on top. Perhaps this is a card to commemorate a marriage or betrothal. The four ladies surrounding her point up, down, and make other gestures. I think they represent the four elements, and the lady in the middle the Quintessence, which was the result of the Rubedo in alchemy (I explain at viewtopic.php?f=12&t=31&start=30#p13823). Ross says (quoted there, from that thread):
There is a canopy over her head. It's tempting to consider it a "marriage" chariot, with all the symbolism that implies (after love, comes marriage, hence chastity/modesty, good qualities in a wife). For what it's worth, Bianca Maria was married in red, "her zodiacal color" (as noted by Pizzagalli - BM was an Aries. Leonello is also noted as considering the color of the planet for the day of the week in choosing his clothing, so symbolism like this might really be present). I don't know if this is Bianca Maria, but the clothing is outstandingly red.
Above, the Catelin Geoffrey is on the right. The groom is Reason, on top is a Saturn-type (as Wisdom, Ficino's theme). Plato in his allegory personifies the horses. In those days, they didn't have Walt Disney's animated animals. To convey the personification aspect, the artist put grooms on or by the horses. On the Cary-Yale, the ignoble horse is on the groom's right and is insolently rearing its front legs. That's how the artist depicted the difference between the two horses, as opposed to the different colors. Maybe the designer had instructed different colors and the artist, seeing the beautiful lady on top, thought it should be two white horses, I don't know. The problem is that there are three messages being conveyed at once in the card: one is of the soul and its three parts; another of the respect, out of fear and modesty, for the beloved, now on top; the third is of a marriage or betrothal commemoration, in which the bridegroom, or perhaps her father if before marriage, traditionally leads the woman to her new home. The CY card assumes the Love card that came before, the heraldics in that card and the whole deck, and a Visconti tradition of betrothal or marriage commemorations in the form of illuminations on paper that refer indirectly to the bride and groom, as Kirsch has amply demonstrated in Five Manuscripts of Galeazzo Visconti, and discussed over many of my posts at viewtopic.php?f=11&t=917.
Now about wings, in reference to the PMB card. Here is where Plato introduces the wings:
And finally about the upper world, where the gods live, and also, once every ten thousand years, the souls of those who are less than gods and who do not have the horses of the gods:The soul in her totality has the care of inanimate being everywhere, and traverses the whole heaven in divers forms appearing--when perfect and fully winged she soars upward, and orders the whole world; whereas the imperfect soul, losing her wings and drooping in her flight at last settles on the solid ground-there, finding a home, she receives an earthly frame which appears to be self-moved, but is really moved by her power; and this composition of soul and body is called a living and mortal creature...The wing is the corporeal element which is most akin to the divine, and which by nature tends to soar aloft and carry that which gravitates downwards into the upper region, which is the habitation of the gods. The divine is beauty, wisdom, goodness, and the like; and by these the wing of the soul is nourished, and grows apace; but when fed upon evil and foulness and the opposite of good, wastes and falls away.
Such beings as these--justice, temperance, and the like--have two noble winged steeds. That is what is pictured on the PMB. The souls that descend to earth are different. From their chariots, they get a view (but not their horses) of such beloved archetypes once in ten thousand years, unless at some point they live the life of a philosopher or a lover inflicted with divine madness. But they have one horse that is ignoble and pulls them down (that's the part that your 16th century illustrators depicted), and in so doing the horses lose their wings and it is only with great difficulty, pain, and resistance that they can get them back.But of the heaven which is above the heavens, what earthly poet ever did or ever will sing worthily? It is such as I will describe; for I must dare to speak the truth, when truth is my theme. There abides the very being with which true knowledge is concerned; the colourless, formless, intangible essence, visible only to mind, the pilot of the soul. The divine intelligence, being nurtured upon mind and pure knowledge, and the intelligence of every soul which is capable of receiving the food proper to it, rejoices at beholding reality, and once more gazing upon truth, is replenished and made glad, until the revolution of the worlds brings her round again to the same place. In the revolution she beholds justice, and temperance, and knowledge absolute, not in the form of generation or of relation, which men call existence, but knowledge absolute in existence absolute; and beholding the other true existences in like manner, and feasting upon them, she passes down into the interior of the heavens and returns home; and there the charioteer putting up his horses at the stall, gives them ambrosia to eat and nectar to drink.
I hope now you can see the relevance of the Phaedrus. One thing about allegories is that they are depicting or describing what cannot be depicted or described, but only hinted at. Plato wasn't writing an ekphrasis, and there are different ways of providing these hints, between language and pictures.