What does If snow be white why then her breasts are dun?

In less flowery speech: "Her breasts are not nearly as white as snow is, in fact, they're really more a sort of brownish color." It's from one of Shakespeare's sonnets, in which he mocks the tendency of poets speaking of a pretty woman to come up with outrageous comparisons that would, if true, be more indicative of a serious medical condition than the height of beauty.