Fausset's Bible Dictionary
argaman , the purple juice of the Tyrian shell fish, Μurex trunculus . (See ; TYRE.) Shani tolaath , an insect color from the cocci or semiglobular bodies as large as a split pea, black but dusted with a grey white powder, on evergreen oaks and other trees. The insect is of the order Ηomoptera , the females have a mouth able to pierce and suck plants. The Arabs call them kermes , from whence come our "caroline" and "crimson". The full grown larva has the dye in greatest abundance. They yield their dye by infusion in water. The dye is fixed by a mordant, anciently alum, now solution of tin.
The double dipping is implied in shunt, differently pointed in Hebrew: Isaiah 1:18, "though your sins be as scarlet (double dyed, deeply fixed so that no tears can wash them away; blood-colored in hue, i.e. of deepest guilt, Isaiah 1:15; the color of Jesus' robe when bearing them, Matthew 27:28) they shall be as white as snow" (Psalms 51:7) (See , DAY OF.) Rahab's scarlet thread was the type (Joshua 2:18). Scarlet was also used in cleansing the leper (Leviticus 14:4). The Mishna says a band of twice dyed scarlet wool tied together the living bird, the hyssop, and the cedar, when dipped into the blood and water. Kurtz makes the scarlet wool symbolize vital health; but Isaiah 1:18 gives a contrary sense. A glaring, gorgeous color (Nahum 2:3); that of the spiritual whore or corrupt church, conformed to that of the beast or God-opposed world power on which she rides (Revelation 17; 18).
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Fausset, Andrew R. Entry for 'Scarlet'. Fausset's Bible Dictionary. https://www.studylight.org/dictionaries/eng/fbd/s/scarlet.html. 1949.