I love this poem. And it's not easy to render into verse though I'm more than confident that you can and will do so.
Here’s my take on this wonderful poem, which might help you sort out some issues of clarity.
Following Chris, purple could be seen to represent arterial blood, which is purplish red or crimson. De la Ceppede’s use of color here is striking and intensely visual, almost like a painting by El Greco, the poet’s contemporary. I think your translation should reflect this vividness. Currently, it is entirely too bloodless to work.
However, there does seem to be a difference between the “rouge” of the coats of arms (“la rouge cottes d’armes”) as well as the redness of sins (“rouges péchés”) and the color crimson. The "rouge" of the coat of arms belongs "appartient" to conquering monarchs, men of power, of this world.
In contrast, “pourpre,” addressed directly over two stanzas, while a variation of red in terms of color, refers only to Christ’s blood. Perhaps it is a kind of Christianized “ichor.” If that goes too far, de la Ceppede indicates “purple” as something different from “red,” in kind if not in hue.
The “gens d’armes” I take to be the mocking soldiers, who killed Christ after they flogged him and gave him a mantle to wear by which, along with the crown of thorns, he is “mocked” as King of the Jews. You have "coat of arms" twice in S1 of your crib.
… Ce roi victorieux
Est justement vêtu par ces moqueurs gens d'armes
D'un manteau, qui le marque et prince, et glorieux.
becomes in my prose translation:
“This victorious king by these mocking soldiers is justly dressed with a mantle that marks him a glorious prince.”
One could also replicate de la Ceppede's striking tmesis with "mark him both prince and glorious."
“Justly” seems appropriate for “justement” because for Christ, not of this world but in it, such a worldly and cruel punishment, though perverse, is just in its way. Since, for believers, the Passion is a part of God’s plan (“par le Père”) for the remission of sins, it is only appropriate that the world of sin mocks and tortures Christ. In this sense, Christ is being "justly" dressed in a bloody mantel by the mocking soldiers.
Perhaps de la Ceppede is inverting the idea of a coat of arms, which is developed in the S4. For Christ, his “escutcheon,” if you will, is his back, which we must assume is drenched in blood after his flogging, and like the monarch’s coat of arms, is also a kind of dark red (crimson) surface or field, but it is one very much different from that worldly red of conquering rulers.
As developed in S4, the French “abimes” like the English “abysses” refers in heraldry to the very center of the shield of a coat of arms. This might tie in nicely with Chris’ observation of sticks/branches (brindelles/brindilles) as a metonym for red lashes across Christ’s bloody back with the "abysses” as the center of an escutcheon. I believe this idea is worth an examination.
Stanza 2 is very striking. The narrator asks purple/crimson to fill his head with its essence to such a degree that it comes out of his eyes onto his beloved Carmelites. For a Christian believer like de la Ceppede, or his other contemporary St. John of the Cross, it would be a blessing to be drenched in the blood of Christ—and not just metaphorically as in this poem!
My overall suggestion is to make this poem as carnal in its import as the original. We need something more graphic. After all, we are talkin’ ‘bout de body ‘n de blood. Make it bleed!
In any case, Lance, I await your revised translation of this poem. I thank you for turning me on to this poet.