As white wool (ως εριον λευκον). Εριον (wool) in N.T. only here and Hebrews 9:19, though old word. The person of the Lord Jesus is here described in language largely from Daniel 7:9 (the Ancient of Days).

White as snow (ως χιων). Just "as snow," also in Daniel 7:9. In N.T. only here and Matthew 28:3.

As a flame of fire (ως φλοξ πυρος). In Daniel 7:9 the throne of the Ancient of Days is φλοξ πυρος, while in Daniel 10:6 the eyes of the Ancient of Days are λαμπαδες πυρος (lamps of fire). See also Revelation 2:18; Revelation 19:12 for this bold metaphor (like Hebrews 1:7).