Wiktionary
a. (context simile said of a human's complexion English) Very white or pale, as if suffering from shock.
Usage examples of "white as a sheet".
You never think it's going to be you: they're looking particularly shut-faced when you go through Clearance and you know it must be Mario because it's the only one running, or you find you can't reach Parkis and you know his operation must have come unstuck because he told you to be here and he doesn't miss an appointment unless the sky's caved in and this time it's poor old Talbot, or you see two of the escape-crew couriers going into Debriefing as white as a sheet and that's either Fitzroy or Crocker and you don't ask anyone which.
Spud, white as a sheet, shook his head, mumbled something and pretended to drop his napkin under the table, so as to keep his frightened face out of sight.
Now he saw: it was Glass, clear as day, and, despite the coloured lights, white as a sheet.
When I had reached the front, the first thing I saw was Granpa lying on the pavement, his head propped up on a box of apples and his face as white as a sheet.
Your face got as white as a sheet, your eyes darted around as though you wanted to run out of the house for dear life then and there.
Meanwhile the port was poured by the butler, who was now as white as a sheet.