to turn white: to become white, or to make something white, most often as the picture of a stain being washed out
Whiteness in the Hebrew Bible is the look of something with nothing on it: fresh snow, washed wool, wood stripped of its bark. This verb names the change into that state. It works in both directions, so a thing can turn white on its own, or someone can make it white.
Only one verse uses it for plain color. Joel describes a fig tree so thoroughly stripped by locusts that the naked branches shine pale. Everywhere else the whiteness stands for a person or a people made clean: scarlet sin turning snow white in Isaiah, a psalmist asking to come out of God's washing whiter than snow, and in Daniel the faithful who pass through persecution and come out bright.
Two things are worth noticing about the picture. Whiteness here is not innocence in the abstract; it is a result, what a stain looks like once it is gone. And it keeps company with the vocabulary of laundry and metalworking: washing, sifting, refining. The plain Hebrew word for "white" (לָבָן) names the shade, and this verb names the change.