Answer: In order to make your plant color up again, you need to keep it growing during the spring and summer months in full sun. It's best to keep it outside and provide it with plenty of moisture and monthly fertilization. Be sure to gradually acclimate the plant to being outdoors.
In the fall, you will have to bring it back inside when the night temperatures drop into the upper forties, setting it in the sunniest spot you have. This should result in lots of color again by the holidays, provided you also somehow keep the plant in total darkness for 12 to 14 hours every night beginning in September. The best way to provide the necessary darkness is to place your plant in a closet every night or cover it with a bucket or cardboard box to block out all light, even from a porch light or streetlight.
This is more effort than most people care to make for a poinsettia, but it can be a lot of fun to see it color up again after all those months.
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