Let the bedsheets stay white. Not to offend you, but the guy looks kinda ridiculous in his concealed form, and a white concealment suit would add to the ridiculous bedsheet-ghost look. Play the ridiculousness up for what it's worth, milk jokes or unease out of it, either way make him a competent villain (unless you already had plans to make him incompetent or something). The contrast between his clumsy looking costume and his highly dangerous competence should create some wonderful dissonance in both the heroes and the reader. Also, make his real form as scary as possible, in stark contrast to the funny looking, ridiculous bedsheet ghost look of his concealed form.
As always, only go with my advice if you find yourself agreeing with it. Making changes that you don't believe in will detract from the quality of your work, regardless of the quality of the advice.
Let the bedsheets stay white. Not to offend you, but the guy looks kinda ridiculous in his concealed
