Answer: Poinsettias need to have short days and consistent uninterrupted night-time darkness in order to change color. It takes about two months or so. Put them in total darkness every night for 14 hours. You can do this by placing them in a closet, under a cardboard box, or in a paper bag, or simply leave them in an unused room where lights are never turned on at night (make sure there is no nearby porchlight or streetlight to shine on them.) They still need to be in a sunny window during the day though. Good luck!
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