Crimson and red are two colors that speak of the guilt of those whose hands were covered with blood (vs. 15). Being covered with blood speaks of extreme iniquity and perversity (cf. Isa. 59:3; Eze. 9:9,10; 23:37,45).
White as snow like wool are substances that are naturally white, and therefore portray what is clean, the blood is guilt having been removed (cf. Ps. 51:7).
Isaiah was a prophet of grace, but forgiveness is not unconditional. It comes through repentance as vs. 19 indicates.