white as a sheet

To be white as a sheet means to become very pale and white because you were startled or frightened. This idiom is referring to a ghost who would be extremely pale, but instead of saying white as a ghost, they say white as a sheet as in the old days when ghost costumes were mad eof white sheets. Used figuratively, it is an expression for someone who seems to have the color drained from their face because they were scared or startled by something.