What is the significance of the red colour of this horse?
Revelation (Apocalypse) 6:4 (ESV)
4And out came another horse, bright red. Its rider was permitted to take peace from the earth, so that people should slay one another, and he was given a great sword.
In Scripture, red is specifically the colour of blood and thus of bloodshed.
4 And out came another horse, bright red. Its rider was permitted to take peace from the earth, so that people should slay one another, and he was given a great sword.