Here's a plein-air study of kids frolicking at the beach by Peder Krøyer. The bits of foam seem whiter than white because in a way, they are.
(Detail of above) By applying the white as a very thick impasto, it sticks up above the rest of the paint, and even casts a little shadow underneath. Since most paintings are illuminated from above, those globs of paint catch a highlight that's actually whiter than ordinary white paint applied flat to the surface of the painting.
Thanks, Timothy Adkins for the photo.