Poinsettia
Choose carefully where you site your Poinsettia to avoid cold drafts from doorways also be careful when siting a plant on a window sill with a radiater below as they do not like the updraft of hot air. A good temperature for your plant will be between 12c and 30c, preferably without to greater fluctuation.
Remember, the quickest way to kill your plant is either to over water it to a point where it is still standing in water one hour later or to under water it to a point where it wilts. Therefore, it is best to aim for a point somewhere between these two conditions. I would suggest feeling the surface of the compost with your fingers daily. It is best to take the little and often approach to remain between these two parameters. Feed on alternate waterings with any good house plant food.
If you are planning to keep your plant long term it will be necessary to repot it into a larger pot early in the new year and to keep it in a position where it can get as much light as posible. Your plant will be happy outside from mid June until the middle of August, when the night temperature begins to fall. At this point, it will be necessary to bring it back inside to a possition with good light (conservatory etc).
Getting the plant to change colour will happen naturally under United Kingdom day length conditions. In order to get the bracts to change colour, in time for the festive season, it will be necessary from the later part of September to ensure that the plant does not get the natural day length extended (ie artifical lights on after nightfall). Basically, 13 hours of light per day the plant will remain green. Thirteen hours of dark per day (everyday)for eight weeks and your plant will be its intended colour.