1:18 scarlet … crimson. The two colors speak of the guilt of those whose hands were “covered with blood” (v. 15). Being covered with blood speaks of extreme iniquity and perversity (cf. 59:3; Eze 9:9, 10; 23:37, 45). white as snow … like wool. Snow and wool are substances that are naturally white, and therefore portray what is clean, the blood-guilt (v. 15) having been removed (cf. Ps 51:7). Isaiah was a prophet of grace, but forgiveness is not unconditional. It comes through repentance as v. 19 indicates.
MacArthur Study Bible
1:18 Ultimate cleansing comes through Christ’s sacrifice (Heb. 10:1–10).
ESV Study Bible
1:18 scarlet. The color depicts hands “full of blood” (v. 15).
white as snow. God can take away the stain of sin without compromising His righteousness because Jesus Christ bore God’s punishment for sinners (53:4–6; Rom. 3:21–26).
Reformation Study Bible