While returning a book to the public library in Dormont, Pa., recently, Anastasia Mustian spotted a flier promoting a new activity there: BYOB coloring-book parties. She snapped a picture of the flier and texted it to several friends.

On the appointed evening, Ms. Mustian grabbed her coat, snatched up a bottle of Cabernet Sauvignon and told her teenage daughter she was heading out to color. “My daughter was like, ‘Really? Coloring?’ ” Ms. Mustian said.

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