They are quite particular about their requirements if you intend to keep them going year round and colour up again. There are some specific amounts of daylight hours they need. The following link should be useful

http://www.gardenaction.co.uk/plantfinder/plantfinder_poinsettia_1.asp

Thanks. Will let you know how I get on.
Don't intend to keep them year-round, though.
Just want them to last through the Christmas season....

be very careful not to get it cold..... taking it home for example make sure you keep it covered I've known them lose their leaves very quickly after a chill, and in a draughtproof place in the house and an even temperature if possible

I note you're in France - I've noticed in recent years that a lot of them die within a week or so of bringing them home over here. Tesco now has a guarantee on theirs saying they'll last till January - even though some of the ones on display were already withering.... I just wait till the last possible moment to get one.