On the Day of Atonement the priests laid aside their usual sacerdotal garments that projected beauty & distinction (Ex. 28.4,36,42) in favor of the more somber and outwardly humble uniform consistent with the weighty occasion of the atonement (Lev. 16.4). The goal was to project grief over sin and the duty of atonement.
So too when the true Melchizedek came he did not come in dazzling festal garments, but instead this great high priest came somberly, as a man of sorrows, acquainted with grief (Is. 53.2-3) as he made atonement for the sins of his sons and daughters, those whose names were carved on his breastplate.