Albert Barnes Commentary Zechariah 6:2-3

"In the first chariot were red horses; and in the second chariot black horses; and in the third chariot white horses; and in the fourth chariot grizzled strong horses." — Zechariah 6:2-3 (ASV)

The symbol is different from that in the first vision. There (Zechariah 1:8), they were horses only, with their riders, to go back and forth to inquire; here they are war-chariots with their horses, to execute God’s judgments, each in their turn. In the first vision also, there is not the characteristic fourfold division, which recalls the four world-empires of Daniel (Daniel 2); after which, in both prophets, the kingdom of Christ is mentioned. Even if the grizzled horses are the same as the speckled of the first vision, the black horses are lacking there, as well as the succession in which they go forth. The only resemblance is that there are horses of various colors, two of which, red and white, are the same.

The symbol of the fourth empire, grizzled, strong, remarkably corresponds with the strength and mingled character of the fourth empire in Daniel.