Verse 8b says, "He was standing among the myrtle trees that were in the bottoms." These myrtle trees signify the humiliated people of Israel in their captivity.
As Christ was riding on a red horse, He was standing among the myrtle trees that were in the bottoms of the valley. This means that He remained strongly among the captured Israel in the lowest part of the valley in their humiliation. Captured Israel was in the lowest part of the valley, and Christ was ready to do anything for them swiftly. Christ, the One on the red horse, was their patron taking care of them in their captivity.
"Behind Him there were red, reddish-brown, and white horses" (v. 8c). Christ was riding on a red horse, and following Him were horses of three different colors. This indicates that Christ's redemption (the red horse) leads the repentant Israel (the reddish-brown horses) to be justified and accepted by God swiftly (the white horses). Although they were the redeemed people of God (the red horses), as the reddish-brown horses indicate, they were not that pure but were a mixture. The word reddish indicates redemption, and the word brown indicates a mixture. Eventually, as signified by the white horses, when the redeemed people of God come to God and are dealt with, they will be justified.
This vision of the horses portrays the situation of Israel in their captivity. In the eyes of God, Christ the Redeemer was with them taking the lead, and they, God's redeemed people, were following Him. Because they were God's redeemed people, they appear at first sight as red horses. But because they were not pure, they are signified also by the reddish-brown horses. They need to contact God and to be dealt with by Him in order to gain God and be justified by Him and thus become those signified by the white horses. Once they repent, they will swiftly be accepted by God and justified by Him.