Can refer to immorality and sin, but also the redeeming blood of Jesus Christ. It is seen as a symbol of royalty and of indulgence. In Kabbalah a scarlet bracelet is worn on the left hand to ward off evil.
See 2 Moses 24:23, Genesis 38:28, Exodus 26:36, 2 Samuel 1:24, Proverbs 31:21, Song of Solomon 4:3, Isaiah 1:18, Zenos 3:7, Matthew 27:28, Revelation 17:3-4.