You know, I consider myself a walking expert on the TdM and I never noticed that before! Yes, it seems to be flesh colored also in the Grimaud version, which I consider (for several reasons) to be the closest we have to the original coloration.
I think Rasa's answer is essentially the correct one: it is an extension of the driver. In other words, the discipline being symbolized (the poetic mysteries, to be exact) entails or enables restraint of the flesh, that is, of the sensual, in deference to something deeper and more lasting. Its number, 7, makes it the symbol of the seven steps of initiation, of alchemy, of the great Mystery (Orphic, as I think of it). It helps us see that this trump represents the Ma'aseh Merkavah, the 'Work of the Chariot', the inner mysteries that preceded Kabbalah and on the basis of which (that is, of the meaning of Ezekiel's wheels) the latter took its being. The flesh color, then, asks us to internalize this trump especially, rather than thinking of it as some equestrian noble passing us by on the street or something.
G.K. Spain, poet-fiddler and inadvertant thread-killer who now mostly just lurks and learns.