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 6:5-6: cf. Zechariah 1:8 - there only red, speckled and white horses were seen; here there are also black horses).