Sonnet 130, "If snow be white, why then her breasts are dun;", What is being compared to what? Is this a positive or negative comparison?

Snow be white could be her skin, and i think it's a negative comparison.

Explanation:

I'm thinking that maybe snow be white can be her skin, meaning her skin is is fair and white like snow, like snow white. Dun means a dull grayish brown color. So they could be saying , if her skin is fair and white like snow then why are her breasts such a dull color. It's most likely a negative comparison. I could be wrong but this is my take on it.

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