No, they were not. That is evident from how John describes what he saw. The horses and their riders merged into one display of chaotic colour illustrated specifically by the horses’ breastplates. The breastplates were intended to protect the horse from the enemy’s sword, spear, or arrow, and so indicated that there is no way to stop these horses. What you saw as these mounted horses attacked was fire (orange), sapphire (blue), and sulfur (yellow).
17 And this is how I saw the horses in my vision and those who rode them: they wore breastplates the color of fire and of sapphire and of sulfur, and the heads of the horses were like lions’ heads, and fire and smoke and sulfur came out of their mouths.