Why Didn't My Poinsettia Plants Turn Red? | Gardening Advice
The red — or pink or white — “leaves” are actually showy bracts around the inconspicuous flowers. They will be more colorful if the plant is given a small dose of high-nitrogen fertilizer every two weeks after color starts appearing.
To get that first appearance of color, though, the plants must be made to flower, and that won’t happen without long nights. To trigger bloom, poinsettias must have 14 hours of complete darkness — consider a closet — every night for eight to ten weeks. Start in mid-September, and you should have bracts in time for the holidays.