Hi there, BL! Purée will give you the best flavor and color in your buttercream, we'd recommend using 1/3 cup (60 grams) of purée. You're welcome to experiment with juice but it's not something we've tried before, we're not sure you'll get much flavor without needing to add a lot which will thin out your buttercream. A raspberry powder made from freeze-dried raspberries could also work! We don't think you'll end up with a gritty frosting as long as you grind the berries very finely and sift out the seeds. We hope this can help and happy baking! 

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