"You are as white as a sheet."
Translation:Você é branco como um lençol.
I believe you're not wrong. Both options are similar, but the exact meaning should be preserved.
AS white as a sheet = tão branco quanto um lençol
white as a sheet = branco como um lençol
The function of "tão" is to state a more exact comparison. "Tanto quanto" and "tão (something) quanto" are quantity comparers, and they state that both quantities are equal.
I'd use it when talking about quantities that can be measured. But that's not an obligation.
To say "ele é tão rico quanto ela" means he and she have the same "amount of richness", their wealths are equivalent.
To say "ele é rico como ela" means they both are rich, but their wealth are not necessarily equal.
To say "tão branco quanto um lençol" means the "level of whitenes" (is there such a thing?) of the person is the same as the sheet's.
While "branco como um lençol" just means the person reminds a sheet because he's too white.
"White as a sheet" is a common expression in English meaning "very pale". The sheet in question is a pure white bed sheet, not a sheet of paper (I think at the time the expression was born most sheets were pure white and kept that way with bleach). There are a few more "white as" expressions listed here: http://idioms.thefreedictionary.com/white+as+a+sheet