I am afraid I can help you but little with Zechariah. There is a difference in the characters of chapters 1. and 6. In chapter 6, we have not men, that is, we have merely the providential agents; in chapter 1 we have angels who stand before the Lord of all the earth, and overrule the working of these agents, though the agents are in view. The man among the myrtle trees, or angel, pleads for Jerusalem. I have thought that the red might be God's judgment. Babylon had been this in the Lord's hand on His people. Then the man upon the red horse was the one who on the Lord's behalf had executed the judgments, and was now using Persia as His instrument of judging, and so favoring the Jewish people.
Abstractions alone meet symbols; the white are not Christ, the sun is not Christ. White are victorious triumph, and so a white horse; it is Christ triumphant, or His enemies, if they are on white horses. The sun is supreme power; when Christ takes it He is supreme power: so with the red or any other. If Babylon was judgment it would be of such a color; if Persia, that also. But I do not pretend to give the sense as an "oracle of God."