Why do poinsettias change colors?

Poinsettias require short days and long nights in order to bloom and for their bracts to turn color. They need total blackness if possible; you don't even want the light from the moon or stars to enter the room where the poinsettias are being kept.

It will take a while but the poinsettias will turn red by doing this.

(http://www.howtodothings.com/home-garden/how-to-make-a-poinsettia-turn-red)