Exodus 28:40

And for Aaron's sons, as members of the order of priests, thou shalt make coats, and thou shalt make for them girdles, and bonnets shalt thou make for them, caps instead of the more elaborate turban of Aaron, for glory and for beauty, to signify both the great dignity and the peculiar excellence of Jehovah's worship. The garments of the ordinary priests were pure white with the exception of the girdle; for the color of purity was exceptionally appropriate in the case of the priests, who were continually engaged in making offerings in behalf of the people.