To fully understand what is happening, it is useful to recognize where various events occur. The apostle John is in the
Temple of God, because the book with the seals are in God's hand (Rev 5:1),
and when Jesus takes it, the 24 elders surrounding God's throne bowed down to Him (Rev 5:8).
When the first seal is broken, a seraph calls out a white horse whose rider is given a bow and crown. The white horse
rider is tasked with conquering and to conquer; he promotes wonton annihilation and subjugation.
This is not the first mention of colored horses and their riders.
In Zechariah's first vision (Zech 1:7-11), white horses follow a red
horse while patrolling the earth and their riders were angels.
In Zechariah's eighth vision (Zech 6:1-8), he sees four chariots, which
are the four spirits of heaven. These four chariots / spirits of heaven are angels
(2 Ki 6:15-17; Ps 68:17;
Isa 66:15). Going forth between standing before God and patrolling the earth, the third
chariot, being pulled by white horses, follows the chariot with black horses to the "north country" to appease God's wrath.
Without much detail, it is difficult to understand Zechariah' prophecies; however, one can gain a better understanding of
the colored horses and their riders who emerge with the breaking of the first four seals, and they all work in concert.