(power of the) flesh, physical existence, bodily presence, etc.
It's used maybe not metaphorically, but as an extension of the usual meaning, not in the usual sense of "skin colour", but to replace and enhance upon chair and its connotations, carrying notions too of incarnation in the sense that you have a spirit and a body, but that here it is the body, separate from the mind and spiritual desire, that works its effect.
"Carnality" captures something of it, but might be a tad too powerful. The "earthly reality" of the young man?
Does qui in the last part (la carnation du jeune homme qui exerce son charme) refer to the young man (as you have translated it) or to his carnation, do you think?
I rather get the impression that the young man is a "victim" of his body, that he does little or nothing to be seductive, but rather that it is his body, his carnation, his earthly reality that people find so overwhelmingly attractive.
Also, I'm tempted to read regard not as "gaze" (or "eyes") which tends to convey the sense that he is "active" in the seductive process, but rather as "features": "it is the young man's earthly reality which exercises its charm/seductive effect (or "seduces") through the desire conveyed through his features ... Again, he is a victim of his own good looks [I know what I'm talking about ;-) ], does not actively seek to please others or gratify his own senses, but because of his body, his looks, his features, his "look" (regard), not "the way he looks at a person", that he is preyed upon. His desire is indeed conveyed by his eyes, but only in the way that our eyes and expressions betray our inner feelings, despite ourselves, or even without us even being (particularly) aware of our feelings.
You have the full context, I may be very far from the mark, but you might like to ponder on it.
Note added at 13 hrs (2011-09-08 13:21:09 GMT)
Re. intended meanings as not defined by dictionaries:
In English too, "carnation" means "a fleshy pink colour" or "flesh tints (in a painting)". The Latin origin means "fleshiness".
Enfin, cette stratification de signes se duplique une ultime fois, puisqu’elle est d’abord offerte à l’œil du Maître avant de l’être à celui de l’amant : une vertigineuse dialectique du simulacre et de l’INCARNATION se joue à la surface de ce visage, qui se DÉRÉALISE POUR MIEUX SE RÉALISER. La présence du troisième œil vient fonder et clôturer le dispositif permettant la transformation du corps en image : articulant autour de lui un espace de représentation, il transforme l’échange des corps en théâtre de cet échange, offrant à M. M une « double incarnation, L’ACTEUR DONNANT SA CHAIR, ET LA SCÈNE LA MAINTENANT AU TITRE DE CHAIR IMAGÉE », et la scène décrite, comme la scène de théâtre, « NE NOUS PRÉSENTE PAS DES IMAGES DE CORPS RÉELS mais plutôt des CORPS DONT LA CHAIR IRRÉALISÉE VIVIFIE LE SUBSTRAT DE L’IMAGE et contribue à sa CARNATION devant le regard du spectateur »
Seems to me this is more about giving "body" to what isn't there than about colouring what isn't there. The former being a prerequisite to the latter in any case! As with the definition with respect to paintings, carnation is the surface finish to something that isn't there but which acquires flesh and body by virtue of that carnation.
The carnations of the painting had withered, but the eyes were still wonderful in their depth and brilliancy of colour. (112-13, je souligne)
 Ce sont les « CARNATIONS » QUI SONT FLÉTRIS, MAIS ON ENTEND BIEN QUE C'EST DE LA CHAIR QU'IL S'AGIT.
Here it's not the colour of the skin that matters, surely, but the skin as the representation of the flesh and carnality within:
Il déboutonne puis reboutonne, feignant dans son souffle, des variations de température, un bouton de sa chemise, comme POUR ME DÉVOILER UN PEU PLUS DE SA CARNATION, comme pour jouer tout doucement avec mon désir.",
Read lines 3 to 6 on page 7 here:
Local time: 13:55
Native speaker of: English
PRO pts in category: 110