This painting draws on imagery from several passages in the book of Revelations including:
I watched as the Lamb opened the first of the seven seals. Then I heard one of the four living creatures say in a voice like thunder, “Come!” I looked, and there before me was a white horse! Its rider held a bow, and he was given a crown, and he rode out as a conqueror bent on conquest.
When the Lamb opened the second seal, I heard the second living creature say, “Come!” Then another horse came out, a fiery red one. Its rider was given power to take peace from the earth and to make people kill each other. To him was given a large sword.
When the Lamb opened the third seal, I heard the third living creature say, “Come!” I looked, and there before me was a black horse! Its rider was holding a pair of scales in his hand. Then I heard what sounded like a voice among the four living creatures, saying, “Two pounds of wheat for a day’s wages, and six pounds of barley for a day’s wages, and do not damage the oil and the wine!”
When the Lamb opened the fourth seal, I heard the voice of the fourth living creature say, “Come!” I looked, and there before me was a pale horse! Its rider was named Death, and Hades was following close behind him. They were given power over a fourth of the earth to kill by sword, famine and plague, and by the wild beasts of the earth.
When he opened the fifth seal, I saw under the altar the souls of those who had been slain because of the word of God and the testimony they had maintained.
Quoted from BibleHub