The following are some cold temperature effects on poinsettias.
Cool temperatures can delay initiation and development of flower parts. The impact of cool temperatures can readily be seen in a greenhouse where temperature control is not uniform and there are cold and hot areas within the greenhouse. Adequate heating facilities and horizontal air flow fans can reduce this variability.
Sales and delivery of plants during periods of extremely cold temperatures can be difficult because poinsettia bracts can be severely damaged by temperatures below 50°F. Red bracts develop a blue to a silver-white color and the extent of discoloration is dependent on length of exposure to such chilling temperatures. In some areas of the country, retail outlets are not accustomed to protecting poinsettias from chilling injury. Sleeving will help prevent chilling injury.
Information from the Poinsettia Problem Diagnostic Key on Physiological Disorders from North Carolina State University http://www.ces.ncsu.edu/depts/hort/poinsettia/corrective/a11.html