The second war of Act 1 sc ii is "bloody." This establishes the imagery of blood and violence seen through the colour red.

There is the Captains comment of how violently and bravely Macbeth fought,

Which smoked with bloody execution...

There is also blood imagery implied in the Captain's following quote:

Like valor’s minion carved out his passage
Till he faced the slave;
Which ne'er shook hands, nor bade farewell to him,
Till he unseamed him from the nave to th' chops,
And fixed his head upon our battlements.