Answer:
It adds a sense of purity almost. In Sylvia Plath's poem Mushrooms, we can see the description of mushrooms growing overnight. They do so gingerly, carefully, quietly, so that no one can see them develop. They do so whitely, which gives us the impression that these mushrooms are beautiful, almost heavenly.
When something is white, it is pure, neat, beautiful, angelic almost. So that's the impression we get from this particular word in the poem.
