white as a sheet
Very pale in the face, as in She was white as a sheet after that near encounter. This simile, dating from about 1600, survives despite the fact that bedsheets now come in all colors.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.