Zechariah 6:2

In the first chariot were red horses; and in the second chariot black horses; In the first chariot were red horses; and in the second chariot black horses;

In the first chariot were red horses - implying carnage.

In the second chariot black horses - representing sorrow; also famine (Revelation 6:5: cf. - there only red, speckled and white horses were seen; here there are also black horses).