The riders wore armor in colors matching their horses' plagues:
red for fire,
blue for smoke, and
yellow for sulfur.
These are signs of judgment in Scripture (see Revelation 14:10–11; 19:20; Genesis 19:24–28; Psalm 11:6; Ezekiel 38:22; Luke 17:29).
The horses resemble the terrifying monsters from Greek stories depicted on ancient buildings and celebrated in old dramas.
A third: See study note on Revelation 8:7–12.