Charles Ellicott Commentary Numbers 19:6

"and the priest shall take cedar-wood, and hyssop, and scarlet, and cast it into the midst of the burning of the heifer." — Numbers 19:6 (ASV)

And the priest shall take cedar wood, and hyssop, and scarlet.— According to the Targum of Jonathan, another priest was to do this, but the reference seems to be to Eleazar.

Cedarwood may be regarded as the emblem of fragrance and incorruption; hyssop as the emblem of purification; and scarlet (or crimson) wool or cloth may be regarded as emblematical both of sin, which is compared to it in Isaiah 1:18, and also of the blood, which is the life, the shedding of which was necessary for the remission of sin.

All of these items were used in the purification of the leper (Leviticus 14:4). In both cases, there appears to be a typical reference to the sprinkling of the blood of Christ .