of fire - the fiery color of the breastplates answering to the fire which issued out of their mouths.
of jacinth - literally, “of hyacinth color,” the hyacinth of the ancients answering to our dark blue iris: thus, their dark, dull-colored breastplates correspond to the smoke out of their mouths.
brimstone - sulphur-colored: answering to the brimstone or sulphur out of their mouths.