Look for plants with a full cluster of cyathia (the true flowers) located in the center of the bracts, which are actually modified leaves and can be a variety of colors. The flowers should be tightly budded. The cyathia holds the pollen for the plant and as soon as it opens and sheds pollen, the leaves will start to fall off, typically the colored ones first. Check the base of the bract. New flowers are green and red-tipped. Mature cyathia are yellow. Poinsettia bracts maintain their color much longer if you select one that is in the beginning stages of flowering.
Tip: Poinsettias are vibrant additions to holiday arrangements. A stems lasts a week or more if you cut it on the diagonal and singe it in a candle flame before placing it in a container.