单选:You know what colors look good --- your skin.

B
against
首先颜色肯定不是在身上,不能用on,例如on the tree,就是长在树上了.排除法 A也不对.
against,在这有相对比较的意思,拿皮肤和颜色相比较.而且一般都是against your skin 一起用
Lips against your skin嘴唇在你的皮肤
Lying soft against your skin轻轻的靠在你身上