You'll never get the plant as red as it was in the garden centre when you bought it.....Poinsettias are often subjected to a gas treatment to bring the colours out and get the plant into colour at the right time. Not too sure what the gas is....could be bromide IIRC.
Ps...have you ever seen these things growing wild ? I saw big 6-8' tall shrubs of poinsettia growing in people's gardens in Barbados.....quite amaizing really considdering they're assosiated with xmas over here !!!