Juice from grapes can have a reddish colour (depending on the variety). But the crop the angel was harvesting was ultimately not grapes but people. The term “the earth” as it appeared in Revelation 14:15 and Revelation 14:18–19 stood for earth’s inhabitants, “every nation and tribe and language and people” (Revelation 14:6). The wages of sin is death (Genesis 2:17; Romans 6:23), and the life of man is in his blood (Leviticus 17:11, Leviticus 17:14). So man’s blood must be shed as result of sin. As the treader in God’s winepress tramps upon harvested humanity, a river of blood flowed from the press of God’s wrath. This judgment is God’s righteous penalty on sin.
20 And the winepress was trodden outside the city, and blood flowed from the winepress, as high as a horse’s bridle, for 1,600 stadia.