DOTLAN EveMaps

The color of the horse; another horse that was red. This symbol cannot be mistaken. As the white horse denoted prosperity, triumph, and happiness, so this would denote carnage, discord, bloodshed. This is clear not only from the nature of the emblem, but from the explanation immediately added: "And power was given to him that sat thereon to take peace from the earth, and that they should kill one another." There is no possibility of mistaing this, that a time of slaughter is denoted by this emblem.

Barnes' Notes on the Bible