I may be mistaken but looking at your picture, the mother feeding the babies does not look Yorkie to me. Am I mistaken? I am so sorry if I am, but the fur does not look like a Yorkie color (black, blue, or tan) fur and also the texture looks curly (maybe a cotton coat?) Are the puppies pure-bred? If they are mixed breed then it is probably not unlikely to have different colored pups.
Sorry if I'm mistaken
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Snick & Viv