The problem in the italicized element in the sentence below is
she was white as a sheet.
trite expression
slang/colloquialism
stilted language
o vague language

Trite expression.

Explanation:

White as a sheet is an example of a simile, and similes are considered to be trite expressions, also known as cliches. Usually, in formal language, such expressions should be avoided - if you can avoid using an idiom, do so in formal expressions. However, they are allowed (and usually welcomed) in literature as they can be quite artistic.

Similes are comparisons between words or phrases using like as a comparing word.