Hey, I've actually been wondering the same thing as well, so I can't honestly say that I know too much about poinsettias. I did find a site with a few basics on poinsettias, and how to get them to change color. (This is directly from the site):
"Q: How do you get a poinsettia to bloom?
A: From the first of October until the colored bracts can be seen it should not receive any light at night. Cover the plant at dusk every evening with a light-proof bag and uncover about 8:00 AM in the morning. A closet may be ideal to put it in at night. Even a quick, short exposure to even dim light can prevent flowering."
Anyways, hope this helps, and best of luck to you!