The young girl dresses all in white in stanza 5 as she prepares to go to church. Not only is the color a symbol of her innocence and purity, but it shows the respect the child has for church. While most young kids might not mind a few stains on their clothing, the young girl in the poem wants to arrive at church looking clean and well-dressed.
Later, in stanza 7, the white shoe is the only thing the mother can recover in the rubble. Visually, that's some striking imagery; were the shoe any other color, it might not stand out so well. Randall wants to leave us with this final image of destroyed purity to emphasize the tragedy of the bombing.