I'm no stranger to this problem, because I often wear my high-necked white sweater when I head out to do a report. No matter how careful I am, it ends up with traces of make-up!

To begin
Place a paper tissue over your face while you put your sweater on. That way, your sweater won't pick up any foundation as you pull it over your head. The best solution - I might even call it my all-purpose stain remover - is whitening toothpaste, the kind sold for dentures. Its action is much stronger than ordinary toothpaste.

Moisten the stain, then rub some toothpaste over it. Let it work for a few minutes, then use an old toothbrush on the stain, brushing with the nap of the fibers. Rinse.