I had the same question while doing my research because this information was not easily available. As I say in the post, it appears to be both genetic and environmental. While the colony can be expected to have similar genetic make up, since the odor of an ant will depend on the food it eats as well as the places it visits, it becomes necessary for the nest members to "groom" each other so that the smells mingle to give rise to a common "colony smell". The sharing of food also seems to be important for the creation of this colony specific smell. The ants apparently store this synthesized colony smell in a gland near their mouths and apply it on their body so that they smell right. This book talks about this phenomenon in case of fire ants (page 252), and mentions that an ant from an alien colony will be accepted with "less aggression" if it is covered in the colony's smell and that a member of the same colony covered in some other colony's smell will be received with hostility. In short, each member apparently contributes to the creation of the smell, and the smell of one ant will be a subset of the mixed colony smell.